[PowerPC] Implement atomic NAND operations as actual NAND

This changes the implementation of atomic NAND operations
from "a & ~b" (compatible with GCC < 4.4) to actual "~(a & b)"
(compatible with GCC >= 4.4).

This is in line with the common-code and ARM back-end change
implemented in r212433.

llvm-svn: 212547