Forcing them to be embedded in makefiles precludes being able to run them in
rustbuild, and adding them to compiletest gives us a great way to leverage
future enhancements to our "all encompassing test suite runner" as well as just
moving more things into Rust.
All tests are still Makefile-based in the sense that they rely on `make` being
available to run them, but there's no longer any Makefile-trickery to run them
and rustbuild can now run them out of the box as well.
Similarly to #31629 where an i586-unknown-linux-gnu target was added, there is
sometimes a desire to compile for x86 Windows as well where SSE2 is disabled.
This commit mirrors the i586-unknown-linux-gnu target and simply adds a variant
for Windows as well.
This is motivated by a recent [Gecko bug][ff] where crashes were seen on 32-bit
Windows due to users having CPUs that don't support SSE2 instructions. It was
requested that we could have non-SSE2 builds of the standard library available
so they could continue to use vanilla releases and nightlies.
[ff]: https://bugzilla.mozilla.org/show_bug.cgi?id=1253202
