[libc] Simplifies multi implementations

This is a roll forward of D101895 with two additional fixes:

Original Patch description:
> 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).

Additional fixes:
 - Fix uninitialized ALL_CPU_FEATURES
 - Use non pseudo microarch as it is only supported from Clang 12 on

Differential Revision: https://reviews.llvm.org/D102233