this also fixes a bug where bootstrap would try to use the fake `rustc` binary built by bootstrap -
cargo puts it in a different directory when using `cargo run` instead of x.py

Same rationale as https://github.com/rust-lang/rust/pull/76544;
it would be nice to make python entirely optional at some point.

This also removes $ROOT as an option for the build directory; I haven't been using it, and like Alex
said in https://github.com/rust-lang/rust/pull/76544#discussion_r488248930 it seems like a
misfeature.

This allows running `cargo run` from src/bootstrap, although that still gives
lots of compile errors if you don't use the beta toolchain.

The majority of the code is only used by either rustbuild or
rustc_llvm's build script. Rust_build is compiled once for rustbuild and
once for every stage. This means that the majority of the code in this
crate is needlessly compiled multiple times. By moving only the code
actually used by the respective crates to rustbuild and rustc_llvm's
build script, this needless duplicate compilation is avoided.

If you have a file in the repository root with the same name as a file
somewhere within a directory, the latter currently won't get formatted.

I have experienced this multiple times and not understood what was
happening; I finally figured out the problem today. This commit fixes
the problem.

First, this reverts the `CFLAGS`/`CXXFLAGS` of #93918. Those flags are
already read by `cc` and populated into `Build` earlier on in the
process. We shouldn't be overriding that based on `CFLAGS`, since `cc`
also respects overrides like `CFLAGS_{TARGET}` and `HOST_CFLAGS`, which
we want to take into account.

Second, this adds the same capability to specify target-specific
versions of `LDFLAGS` as we have through `cc` for the `C*` flags:
https://github.com/alexcrichton/cc-rs#external-configuration-via-environment-variables

Note that this also necessitated an update to compiletest to treat
CXXFLAGS separately from CFLAGS.

It is currently hard coded to llvm if enabled and cranelift otherwise.
This made some sense when cranelift was the only alternative codegen
backend. Since the introduction of the gcc backend this doesn't make
much sense anymore. Before this PR bootstrapping rustc using a backend
other than llvm or cranelift required changing the source of
rustc_interface. With this PR it becomes a matter of putting the right
backend as first enabled backend in config.toml.

Previously it would be skipped if codegen-backends doesn't contain llvm.

We started using it in https://github.com/rust-lang/rust/pull/94023.

Metadata::modified works in all platforms supported by the filetime
crate. This changes brings rustbuild a tiny bit closer towards dropping
the filetime dependency.

It seems to be unused and it is incorrect for arm/aarch64 anyway.

This directory is required for building LLVM now.

Fixes #70468. Closes #89983.

This tidies up the logic in `src/bootstrap/native.rs` such that:

 - `CMAKE_*_LINKER_FLAGS` is not overridden if we add to it twice.
 - `CMAKE_*_FLAGS` also include the standard `*FLAGS` environment
   variables, which CMake respects when we _don't_ set `CMAKE_*_FLAGS`.
 - `llvm.ldflags` from `config.toml` appends to the ldflags Rust's
   bootstrap logic adds, rather than replacing them.

Fixes #93880.

This reverts commit e7cc3bdd, reversing
changes made to 734368a2.

This was only used for the inclusion of 'current' dates into our manpages, but
it is not clear that this is practically necessary. The manpage is essentially
never updated, and so we can likely afford to keep a manual date in these files.
It also seems possible to just omit it, but that may cause other tools trouble,
so avoid doing that for now.

Signed-off-by: NMuhammad Falak R Wani <falakreyaz@gmail.com>

Previously, rustbuild would give strange errors if you tried to reuse the same build directory under two names:

```
$ mkdir tmp && cd tmp
$ ../x.py check
Building rustbuild
    Finished dev [unoptimized] target(s) in 35.27s
Checking stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
^C
$ cd ..
$ mv tmp/build build
$ ./x.py check
Building rustbuild
   Compiling bootstrap v0.0.0 (/home/jnelson/rust-lang/rust/src/bootstrap)
    Finished dev [unoptimized] target(s) in 11.18s

failed to execute command: "/home/jnelson/rust-lang/rust/tmp/build/x86_64-unknown-linux-gnu/stage0/bin/rustc" "--target" "x86_64-unknown-linux-gnu" "--print" "target-libdir"
error: No such file or directory (os error 2)
```

This fixes the error. Reusing the same build directory is useful if you want to test path-things in
bootstrap itself, without having to recompile it each time.

For good measure, this also reruns the build script when PATH changes.

I missed this in https://github.com/rust-lang/rust/pull/89757; it made
`x.py test src/bootstrap` very slow.

pretty_assertions 0.6 -> 0.7, to drop some `ansi_term` version
futures 0.1.29 -> 0.1.31, backported some [fixes](https://github.com/rust-lang/futures-rs/compare/0.1.29...0.1.31) to old verions
futures-* 0.3.12 -> 0.3.19, to remove `proc-macro-hack`, `proc-macro-nested` and fix some [issues](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md#0319---2021-12-18). There exist 0.3.21, but it's quite new (06.02.22), so not updated to.
itertools 0.9 -> 0.10 for rustdoc, will be droppped when rustfmt will bump `itertools` version
linked-hash-map 0.5.3 -> 0.5.4, fix [UB](https://github.com/contain-rs/linked-hash-map/pull/106)
markup5ever 0.10.0 -> 0.10.1, internally drops `serde`, reducing [build time](https://github.com/servo/html5ever/commit/3afd8d63853627e530b3063b0185eea3732cc29f#diff-4c20e8293515259c0aa26932413a55a334aa5f2b37de5a5adc92a2186f632606) for some usecases
mio 0.7.13 -> 0.7.14 fix [unsoundness](https://github.com/tokio-rs/mio/compare/v0.7.13...v0.7.14)
num_cpus 1.13.0 -> 1.13.1 fix parsing mountinfo and other [fixes](https://github.com/seanmonstar/num_cpus/compare/v1.13.0...v1.13.1)
openssl-src 111.16.0+1.1.1l -> 111.17.0+1.1.1m fix CVE-2021-4160

Add a convenient way to inject rustc flags somewhere late in the bootstrap pipeline without causing rebuilds

Useful for -Ztreat-err-as-bug

This is particularly intended for invoking compiletest; the command line there
is long (3,350 characters on my system) and takes up a lot of screen real estate
for little benefit to the majority of those running bootstrap. This moves
printing it to verbose mode (-v must be passed) which means that it's still
possible to access when needed for debugging.

The main downside is that CI logs will by-default become less usable for
debugging (particularly) spurious failures, but it is pretty rare for us to
really need the information there -- it's usually fairly obvious what is being
run with a little investigation.

x.py has support for excluding some steps from the invocation, but
unfortunately that's not granular enough: some steps have the same name
in different modules, and that prevents excluding only *some* of them.

As a practical example, let's say you need to run everything in `./x.py
test` except for the standard library tests, as those tests require IPv6
and need to be executed on a separate machine. Before this commit, if
you were to just run this:

    ./x.py test --exclude library/std

...the execution would fail, as that would not only exclude running the
tests for the standard library, it would also exclude generating its
documentation (breaking linkchecker).

This commit adds support for an optional module annotation in --exclude
paths, allowing the user to choose which module to exclude from:

    ./x.py test --exclude test::library/std

This maintains backward compatibility, but also allows for more ganular
exclusion. More examples on how this works:

| `--exclude`         | Docs    | Tests   |
| ------------------- | ------- | ------- |
| `library/std`       | Skipped | Skipped |
| `doc::library/std`  | Skipped | Run     |
| `test::library/std` | Run     | Skipped |

Note that the new behavior only works in the `--exclude` flag, and not
in other x.py arguments or flags yet.

Building a dozen separate regexps for each test in compiletest consumes significant amounts of CPU cycles.
Using `RUSTC_FORCE_INCR_COMP_ARTIFACT_HEADER` stabilizes hashes calcuated for the individual tests so
no test-dependent normalization is needed. Hashes for the standard library still change so some
normalizations are still needed.