Add missing avx512vl intrinsics for f32->u32 conversions

[Shnatsel]

Fixes rust-lang/rust#158196

Adds 6 intrinsics that were apparently forgotten by Intel and not included into the Intrinsics Guide web viewer, despite being present in the Intel Software Development Manual and implemented in clang. They all lower into a single hardware instruction and there is no other way to access that instruction; the AVX2 emulation is very costly compared to these native conversions.

The implementation mirrors both the existing 512-bit-wide form and the clang implementation for these 128-bit and 256-bit forms.

This is my first contribution so I have no idea what I'm doing. I don't know what the stabilization process for these would be and if these are supposed to be instantly stable or not, so I just made up a new unstable feature name.

[rustbot]

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