[PowerPC] Provide some P8-specific altivec overloads for P7

This adds additional support for XL compatibility. There are a number
of functions in altivec.h that produce a single instruction (or a
very short sequence) for Power8 but can be done on Power7 without
scalarization. XL provides these implementations.
This patch adds the following overloads for doubleword vectors:
vec_add
vec_cmpeq
vec_cmpgt
vec_cmpge
vec_cmplt
vec_cmple
vec_sl
vec_sr
vec_sra