libc: krait: Implement optimized versions of memmove and bcopy

Restore a jellybean optimization for memmove and bcopy on krait.
Code has been refactored to thumb2 for consistency with the rest of
bionic libc, as well as performance and correctness.

Change-Id: I8f2f77dce4534fbce1bdf0188ab353bf34ab8637
Merge remote-tracking branch 'Slim/kk4.4' into slim4.4