Rollup merge of #137037 - RalfJung:x86-sse2-abi, r=workingjubilee

add x86-sse2 (32bit) ABI that requires SSE2 target feature

This is the first commit of rust-lang/rust#135408:

The primary goal of this is to make SSE2 required for our i686 targets (at least for the ones that use Pentium 4 as their baseline), to ensure they cannot be affected by rust-lang/rust#114479. This has been MCPd in rust-lang/compiler-team#808, and is tracked in rust-lang/rust#133611.

We do this by defining a new ABI that these targets select, and making SSE2 required by the ABI (that's the first commit). That's kind of a hack, but it is the easiest way to make a target feature required via the target spec. In a follow-up change (rust-lang/rust#135408), we can actually make use of SSE2 for the ABI, but that is running into some infrastructure issues.

r? `@workingjubilee`

try-job: aarch64-apple
try-job: aarch64-gnu
try-job: aarch64-gnu-debug
try-job: test-various
try-job: x86_64-gnu-nopt
try-job: dist-i586-gnu-i586-i686-musl

Author: workingjubilee

tests/pass/shims/x86/intrinsics-x86-pause-without-sse2.stderr

@@ -0,0 +1,5 @@
+warning: target feature `sse2` must be enabled to ensure that the ABI of the current target can be implemented correctly
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+