[libc] Allow customization of memcpy via flags.

 - Adds LLVM_LIBC_IS_DEFINED macro to libc/src/__support/common.h
 - Adds a few knobs to memcpy to help with experimentations:
   - LLVM_LIBC_MEMCPY_X86_USE_ONLY_REPMOVSB replaces the implementation with a single call to rep;movsb
   - LLVM_LIBC_MEMCPY_X86_USE_REPMOVSB_FROM_SIZE customizes where the usage of rep;movsb

Differential Revision: https://reviews.llvm.org/D94692