[libc] Switch to new implementation of mem* functions

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 (b3f1d58a)

Differential Revision: https://reviews.llvm.org/D136595