libc: krait: Fix memmove half-word atomicity

In tailing case of less than 7, if even number of bytes are left, then
use half word load/store to make sure the whole routine is half-word
atomic

Change-Id: I54948ac050ff866986a7e144d4240652981ea87a
Merge remote-tracking branch 'cyanogen/cm-11.0' into cm-11.0