[libc] Some clean work with memmove.

- Replace `move_byte_forward()` with `memcpy`. In `memcpy` implementation,
it copies bytes forward from beginning to end. Otherwise, `memmove` unit
tests will break.
- Make `memmove` unit tests work.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D109316

GitOrigin-RevId: b659b789c03ac339e28d7b91406b67bb887a426d
3 files changed