Implement AVX512_FP16

[sayantn]

This PR adds the AVX512_FP16 intrinsics in Rust. These intrinsics will be behind the feature gate #[feature(stdarch_x86_avx512_f16)] (rust-lang/rust#127213).

Progress:

• [x] Set intrinsics
• [ ] Arithmetic operations (Addition, Multiplication, Complex Multiplication)
• [ ] FMA
• [x] Cast
• [x] Compare
• [ ] Convert
• [ ] Math (Reciprocal, Sqrt, RSqrt, Max, Min, Reduce)
• [ ] FP-Support (FPClass, getexp, getmant, scale)
• [x] Load-Store
• [ ] Complex number operations

Unsolved Questions

• The LLVM intrinsics for cmp operations need i1. Can we implement these without inline asm?
• Some set intrinsics input the C type _Float16 _Complex. How do we represent that in Rust?

[rustbot]

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[tgross35]

Have you run into any weird behavior with these, or do things seem to be working smoothly? (ignoring the ABI issue for system function calls, that is)

[sayantn]

No problems yet, just that simd_fabs doesn't accept a f16 argument, so i will just use an and operation. I am actively avoiding doing f16 operations in rust, but that's not a blocker for sure.

[bors]

:umbrella: The latest upstream changes (presumably 3dd9579da33db7fc9db68eb36969d387dd706517) made this pull request unmergeable. Please resolve the merge conflicts.

[sayantn]

cc @tgross35 @beetrees