[libc] Add strxfrm Implementation

Introduce `strxfrm` and unit tests. The current implementation is
introduced without locale support.

The simplified function performs a `memcpy` if the `n` value is large
enough to store the source len + '\0', otherwise `dest` is unmodified.

Ticket: https://fxbug.dev/124217

Differential Revision: https://reviews.llvm.org/D147478