Add support for the new [AVX-512-BF16](https://en.wikipedia.org/wiki/AVX-512#BF16) x86 extension for `bf16` conversions. Will need to be added to core::arch. Extension intrinsic docs: https://www.intel.com/content/www/us/en/develop/documentation/cpp-compiler-developer-guide-and-reference/top/compiler-reference/intrinsics/intrinsics-for-avx-512-bf16-instructions.html