powerpc: Optimized st{r,p}cpy for POWER8/PPC64

powerpc: Optimized st{r,p}cpy for POWER8/PPC64

This patch adds an optimized POWER8 strcpy using unaligned accesses.
For strings up to 16 bytes the implementation first calculate the
string size, like strlen, and issues a memcpy.  For larger strings,
source is first aligned to 16 bytes and then tested over a loop that
reads 16 bytes am combine the cmpb results for speedup.  Special case is
added for page cross reads.

It shows 30%-60% improvement over the optimized POWER7 one that uses
only aligned accesses.