security/nss: fix build on powerpc64le

LLVM's integrated assembler can't build ppc-gcm.s:
ppc-gcm.s:642:24: error: invalid memory operand

std          31,-8(SP);

Use as from binutils on powerpc64le.

While here correct the architecture check to also check for Linux's ppc64 - similar to the OS check in the same patch.