[libc] Adding memcpy implementation for x86_64

Summary:
The patch is not ready yet and is here to discuss a few options:
 - How do we customize the implementation? (i.e. how to define `kRepMovsBSize`),
 - How do we specify custom compilation flags? (We'd need `-fno-builtin-memcpy` to be passed in),
 - How do we build? We may want to test in debug but build the libc with `-march=native` for instance,
 - Clang has a brand new builtin `__builtin_memcpy_inline` which makes the implementation easy and efficient, but:
   - If we compile with `gcc` or `msvc` we can't use it, resorting on less efficient code generation,
   - With gcc we can use `__builtin_memcpy` but then we'd need a postprocess step to check that the final assembly do not contain call to `memcpy` (unlikely but allowed),
   - For msvc we'd need to resort on the compiler optimization passes.

Reviewers: sivachandra, abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits, courbet

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D74397