powerpc: Optimized strncmp for POWER8/PPC64

This patch adds an optimized POWER8 strncmp.  The implementation focus
on speeding up unaligned cases follwing the ideas of power8 strcmp.

The algorithm first check the initial 16 bytes, then align the first
function source and uses unaligned loads on second argument only.
Aditional checks for page boundaries are done for unaligned cases
(where sources alignment are different).