[PPC] support for arithmetic builtins in the FE
This adds various overloads of the following builtins to altivec.h:
vec_neg vec_nabs vec_adde vec_addec vec_sube vec_subec vec_subc
Note that for vec_sub builtins on 32 bit integers, the semantics is similar to
what ISA describes for instructions like vsubecuq that work on quadwords: the
first operand is added to the one's complement of the second operand. (As
opposed to two's complement which I expected).