Remove some random unnecessary lint `allow`s

This is duplicated in a few locations throughout the sysroot to work
around issues with not exporting a macro in libstd but still wanting it
available to sysroot crates to define blocks. Nowadays though we can
simply depend on the `cfg-if` crate on crates.io, allowing us to use it
from there!

The files src/libstd/sys/sgx/*.rs are mostly copied/adapted from
the wasm target.

This also updates the dlmalloc submodule to the very latest version.

Co-authored-by: nikomatsakis

This permits easier iteration without having to worry about warnings
being denied.

Fixes #49517

All musl targets except mips[el] assume static linking by default. This
can be confusing
https://users.rust-lang.org/t/static-cross-compiled-binaries-arent-really-static/6084

When the musl/mips[el] targets was
[added](https://github.com/rust-lang/rust/pull/31298), dynamic linking
was chosen because of binary size concerns, and probably also because
libunwind
[didn't](https://users.rust-lang.org/t/static-cross-compiled-binaries-arent-really-static/6084/8)
supported mips.

Now that we have `crt-static` target-feature (the user can choose
dynamic link for musl targets), and libunwind
[6.0](https://github.com/llvm-mirror/libunwind/commits/release_60) add
support to mips, we do not need to assume dynamic linking.

This commit adds a new target to the compiler: wasm32-unknown-unknown. This
target is a reimagining of what it looks like to generate WebAssembly code from
Rust. Instead of using Emscripten which can bring with it a weighty runtime this
instead is a target which uses only the LLVM backend for WebAssembly and a
"custom linker" for now which will hopefully one day be direct calls to lld.

Notable features of this target include:

* There is zero runtime footprint. The target assumes nothing exists other than
  the wasm32 instruction set.
* There is zero toolchain footprint beyond adding the target. No custom linker
  is needed, rustc contains everything.
* Very small wasm modules can be generated directly from Rust code using this
  target.
* Most of the standard library is stubbed out to return an error, but anything
  related to allocation works (aka `HashMap`, `Vec`, etc).
* Naturally, any `#[no_std]` crate should be 100% compatible with this new
  target.

This target is currently somewhat janky due to how linking works. The "linking"
is currently unconditional whole program LTO (aka LLVM is being used as a
linker). Naturally that means compiling programs is pretty slow! Eventually
though this target should have a linker.

This target is also intended to be quite experimental. I'm hoping that this can
act as a catalyst for further experimentation in Rust with WebAssembly. Breaking
changes are very likely to land to this target, so it's not recommended to rely
on it in any critical capacity yet. We'll let you know when it's "production
ready".

---

Currently testing-wise this target is looking pretty good but isn't complete.
I've got almost the entire `run-pass` test suite working with this target (lots
of tests ignored, but many passing as well). The `core` test suite is still
getting LLVM bugs fixed to get that working and will take some time. Relatively
simple programs all seem to work though!

---

It's worth nothing that you may not immediately see the "smallest possible wasm
module" for the input you feed to rustc. For various reasons it's very difficult
to get rid of the final "bloat" in vanilla rustc (again, a real linker should
fix all this). For now what you'll have to do is:

    cargo install --git https://github.com/alexcrichton/wasm-gc
    wasm-gc foo.wasm bar.wasm

And then `bar.wasm` should be the smallest we can get it!

---

In any case for now I'd love feedback on this, particularly on the various
integration points if you've got better ideas of how to approach them!

Fixes #41701.

Use libgcc_s when linking dynamically. Convert the static libunwind to
static-nobundle, as libunwind.a is copied from musl_root and available
in the library search path.

Make sure we link to the static libpthread, so that compiled Rust binaries do not depend on winpthread1.dll.

Historically this was done to accommodate bugs in lints, but there hasn't been a
bug in a lint since this feature was added which the warnings affected. Let's
completely purge warnings from all our stages by denying warnings in all stages.
This will also assist in tracking down `stage0` code to be removed whenever
we're updating the bootstrap compiler.

This commit is an implementation of [RFC 1513] which allows applications to
alter the behavior of panics at compile time. A new compiler flag, `-C panic`,
is added and accepts the values `unwind` or `panic`, with the default being
`unwind`. This model affects how code is generated for the local crate, skipping
generation of landing pads with `-C panic=abort`.

[RFC 1513]: https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md

Panic implementations are then provided by crates tagged with
`#![panic_runtime]` and lazily required by crates with
`#![needs_panic_runtime]`. The panic strategy (`-C panic` value) of the panic
runtime must match the final product, and if the panic strategy is not `abort`
then the entire DAG must have the same panic strategy.

With the `-C panic=abort` strategy, users can expect a stable method to disable
generation of landing pads, improving optimization in niche scenarios,
decreasing compile time, and decreasing output binary size. With the `-C
panic=unwind` strategy users can expect the existing ability to isolate failure
in Rust code from the outside world.

Organizationally, this commit dismantles the `sys_common::unwind` module in
favor of some bits moving part of it to `libpanic_unwind` and the rest into the
`panicking` module in libstd. The custom panic runtime support is pretty similar
to the custom allocator support with the only major difference being how the
panic runtime is injected (takes the `-C panic` flag into account).

Back in 9bc8e6d1 the linking of rlibs changed to using the `link_whole_rlib`
function. This change, however was only intended to affect dylibs, not
executables. For executables we don't actually want to link entire rlibs because
we want the linker to strip out as much as possible.

This commit adds a conditional to this logic to only link entire rlibs if we're
creating a dylib, and otherwise an executable just links an rlib as usual. A
test is included which will fail to link if this behavior is reverted.

Fixes #30535

The only way to get a value for a zero-sized type is `undef`, so
there's really no point in actually having a return type other than
void for such types. Also, while the comment in return_type_is_void
mentioned something about aiding C ABI support, @EddyB correctly
pointed out on IRC that there is no such thing as a zero-sized type in
C. And even with clang, which allows empty structs, those get
translated as void return types as well.

Fixes #28766

Resolve them in this order as well.