Fix perf regression on Cortex A15 by optimized memcpy for Cortex A9

The new Cortex A9 optimized memcpy included an arm-only version that is
slower than the current neon Cortex A15. This arm-only version was used
when the dst was aligned to an 8 byte boundary, otherwise, the original
neon only version was used. This change modifies the code so it always
uses the neon only version for the Cortex A15. It also fixes the use of
a define removed with the previous optimization.

Change-Id: I971527568a177218cd8b76a8bd160aaaf6ea1e42