The new framework makes it explicit which processor feature is being
used and allows for easier per platform customization:
- ARM cpu now uses trivial implementations to reduce code size.
- Memcmp, Bcmp and Memmove have been optimized for x86
- Bcmp has been optimized for aarch64.
This is a reland of https://reviews.llvm.org/D135134 (b3f1d58)
Differential Revision: https://reviews.llvm.org/D136595