Auto merge of #141309 - RalfJung:x86-simd-abi, r=tgross35,nikic,workingjubilee

x86 (32/64): go back to passing SIMD vectors by-ptr

Fixes rust-lang/rust#139029 by partially reverting rust-lang/rust#135408 and going back to passing SIMD vectors by-ptr on x86. Sadly, by-val confuses the LLVM inliner so much that it's not worth it...

Also fixes rust-lang/rust#141848 by no longer actually using vector registers with the "Rust" ABI.

r? `@tgross35`
Cc `@nikic`

try-job: `test-various*`
try-job: dist-i586-gnu-i586-i686-musl
try-job: x86_64-gnu-nopt
try-job: `x86_64-msvc*`
try-job: `i686-msvc*`

Author: bors