[libc] Add memmove implementation.

Use `memcpy` rather than copying bytes one by one, for there might be large
size structs to move.

Reviewed By: gchatelet, sivachandra

Differential Revision: https://reviews.llvm.org/D93195

GitOrigin-RevId: 2423ec583761304f93b5d01493a26aeb11cb5b8f
7 files changed