PowerPC: strncat optimization for PPC64

The optimization is achieved by following techniques:
1. Doubleword aligned memory access and compares using
   cmpb instruction.
2. Loop unrolling for byte load/store.
3. CPU pre-fetch to avoid cache miss.

Backport of ba9cc0714e58a9e8fa73cf6b0e205cbf1e6b71f2