Add support for the RDPRU instruction on Zen2 processors.
User-facing features:
- Clang option -m[no-]rdpru to enable/disable the feature
- Support is implicit for znver2/znver3 processors
- Preprocessor symbol __RDPRU__ to indicate support
- Header rdpruintrin.h to define intrinsics
- "rdpru" mnemonic supported for assembler code
Internal features:
- Clang builtin __builtin_ia32_rdpru
- IR intrinsic @llvm.x86.rdpru
Differential Revision: https://reviews.llvm.org/D128934
Paul Robinson