(Partially) Stabilize AVX512-FP16

[sayantn]

A total of 897 functions, except for these 44 that explicitly use f16 in the signature

• _mm{,256,512}_{set,set1,setr}_ph
• _mm_set_sh
• _mm{,256,512}_{load,loadu,store,storeu}_ph
• _mm_{load,mask_load,maskz_load,store,mask_store}_sh
• _mm{,256,512}_reduce_{add,mul,min,max}_ph
• _mm{,256,512}_cvtsh_h
• _mm{,256}_bcstnesh_ps

Tracking issue: rust-lang/rust#127213

I also changed the feature parameter for these stabilized functions to be stdarch_x86_avx512fp16, as rustc doesn't allow a stable and unstable feature to have the same name

cc @tgross35

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[sayantn]

It would probably be best to rename the stdarch_x86_avx512fp16_2 or something like that rather than having the only difference be an underscore. But that can happen separately.

The other feature name is stdarch_x86_avx512_f16 (not fp16), but I get your point lol