These headers used to be guarded only on PowerPC64 Linux or FreeBSD, but
they can also be enabled for AIX OS target since it's big-endian ready.
Reviewed By: shchenz
Differential Revision: https://reviews.llvm.org/D129461
Previous test in ppc-pmmintrin.c did not check IR of intrinsic function
definition. Add them and simplify.
These tests shouldn't be auto-generated, because we don't want to check
wrapper functions.
Qiu Chaofan