[libc][bug] Fix out of bound write in memcpy w/ software prefetching (#90591)

This patch adds tests for `memcpy` and `memset` making sure that we
don't access buffers out of bounds. It relies on POSIX `mmap` /
`mprotect` and works only when FULL_BUILD_MODE is disabled.

The bug showed up while enabling software prefetching.
`loop_and_tail_offset` is always running at least one iteration but in
some configurations loop unrolled prefetching was actually needing only
the tail operation and no loop iterations at all.