[libc] New version of the mem* framework

This version is more composable and also simpler at the expense of being more explicit and more verbose.
This patch is not meant to be submitted but gives an idea of the change.
Codegen can be checked in https://godbolt.org/z/6z1dEoWbs by removing the "static inline" before individual functions.

Unittests are coming.

Suggested review order:
 - utils
 - op_base
 - op_builtin
 - op_generic
 - op_x86 / op_aarch64
 - *_implementations.h

Differential Revision: https://reviews.llvm.org/D135134