Switch to Arm Optimized Routines for mem* & str*

These are the updated version of the older Cortex Strings Library we
previously used. The Arm Optimized Routines also support CPU features
that are currently in development on FreeBSD, e.g. Branch Target
Identification (BTI). Rather than add BTI support to the old code it's
easier to just use the maintained version.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32774