Add support for AVX-512-BF16 hardware intrinsics

[VoidStarKat]

Add support for the new 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