Convert __mmask16 to use an unsigned type

[Daniel-B-Smith]

To be consistent with Clang:

https://github.com/llvm/llvm-project/blob/1b02db52b79e01f038775f59193a49850a34184d/clang/lib/Headers/avx512fintrin.h#L37

and to simplify literals where you want to set the top bit to 1.

[rust-highfive]

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[Amanieu]

Thanks!