PowerPC: Optimized mpn functions for PowerPC64

This patch add optimized __mpn_addmul, __mpn_addsub, __mpn_lshift, and
__mpn_mul_1 implementations for PowerPC64. They are originally from GMP
with adjustments for GLIBC.
PowerPC: Optimized mpn functions for PowerPC64/POWER7

This patch add optimized __mpn_add_n/__mpn_sub_n for PowerPC64/POWER7.
They are originally from GMP with adjustments for GLIBC.