[libc] Simplifies multi implementations and benchmarks

This is a follow up on D101524 which:
 - simplifies cpu features detection and usage,
 - flattens target dependent optimizations so it's obvious which implementations are generated,
 - provides an implementation targeting the host (march/mtune=native) for the mem* functions,
 - makes sure all implementations are unittested (provided the host can run them),
 - makes sure all implementations are benchmarkable (provided the host can run them).

Differential Revision: https://reviews.llvm.org/D101895