TIMINGSAFE_MEMCMP: adding this new libc function, inspired from OpenBSD's version.

Signed-off-by: Shawn Webb <lattera@gmail.com>
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/userlandenhanced
Merge remote-tracking branch 'origin/hardened/current/userlandenhanced' into hardened/current/unstable