PowerPC: optimized memmove for POWER7/PPC64

This patch adds an optimized memmove optimization for POWER7/powerpc64.
Basically the idea is to use the memcpy for POWER7 on non-overlapped
memory regions and a optimized backward memcpy for memory regions
that overlap (similar to the idea of string/memmove.c).

The backward memcpy algorithm used is similar the one use for memcpy for
POWER7, with adjustments done for alignment.  The difference is memory
is always aligned to 16 bytes before using VSX/altivec instructions.