libc: krait: Re-factor high-performance memcpy for thumb2

The majority of libc under bionic is built for thumb2.  Refactor the
high performance memcpy used in previous builds for thumb2, including
information that can be used for stack-unwinding.

Change-Id: Ib5f7ab354f39313758402ec02b0aea27b15d45fa
Merge remote-tracking branch 'cm/cm-10.2' into cm-10.2