Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

Replaces #61544
Fixes #59917

We need CXX to build llvm-libunwind which can be enabled for all
targets.
As we needed it for all hosts anyways, just move the detection so that
it is ran for all targets (which contains all hosts) instead.
Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

This extends a test in the previous commit to assert that we don't build
extra rustc compilers even when the "extended" option is set to true.
This involved some internal refactoring to have more judicious usage of
`compiler_for`, added in the previous commit, as well.

Various `dist::*` targets were refactored to be parameterized with a
`Compiler` instead of a `stage`/`host`, and then the various parameters
within the `Extended` target were tweaked to ensure that we don't ever
accidentally ask for a stage2 build compiler when we're distributing
something.

This commit furthers the previous one to ensure that we don't build an
extra stage of the compiler in CI. A test has been added to rustbuild to
ensure that this doesn't regress, and then in debugging this test it was
hunted down that the `dist::Std` target was the one erroneously pulling
in the wrong compiler.

The `dist::Std` step was updated to instead account for the "full
bootstrap" or not flag, ensuring that the correct compiler for compiling
the final standard library was used. This was another use of the
`force_use_stage1` function which was in theory supposed to be pretty
central, so existing users were all evaluated and a new function,
`Builder::compiler_for`, was introduced. All existing users of
`force_use_stage1` have been updated to use `compiler_for`, where the
semantics of `compiler_for` are similar to that of `compiler` except
that it doesn't guarantee the presence of a sysroot for the arguments
passed (as they may be modified).

Perhaps one day we can unify `compiler` and `compiler_for`, but the
usage of `Builder::compiler` is so ubiquitous it would take quite some
time to evaluate whether each one needs the sysroot or not, so it's
hoped that can be done in parallel.

Uses relative libdir to place libraries on all stages.
Adds verbose installation output.

This allows us to e.g. test compiletest, including doctests, in stage 0
without building a fresh compiler and rustdoc.

The RUSTDOC_LIBDIR should be rustc_libdir, not sysroot_libdir; rustdoc
is like the compiler and should link against rustc's libdir.

Some people currently (i.e., in general, may not be on master) have doc
tests working, but no attempt to determine why has been attempted.

Using CC, CFLAGS, CXX, CXXFLAGS, AR and RANLIB breaks cross compilation
because host is built first and has correct values. The same
values are incorrect for the target however.

This commit works around the newly-introduced LLVM shared library.

This is needed such that llvm-config run from
librustc_llvm's build script can correctly locate it's own LLVM, not the
one in stage0/lib. The LLVM build system uses the DT_RUNPATH/RUNPATH
header within the llvm-config binary, which we want to use, but because
Cargo always adds the host compiler's "libdir" (stage0/lib in our
case) to the dynamic linker's search path, we weren't properly finding
the freshly-built LLVM in llvm/lib. By restoring the environment
variable setting the search path to what bootstrap sees, the problem is
resolved and librustc_llvm correctly links and finds the appropriate
LLVM.

Several run-make-fulldeps tests are also updated with similar handling.

Use `clear_if_dirty` on std for backend changes, just as we do for
changes to rustc itself, so new codegen is correctly applied to all
later compiler stages.

Fixes #48298.