mpi/longlong: update powerpc macros from GCC

* mpi/longlong.h [__powerpc__, __powerpc64__]: Update macros.

Update longlong.h powerpc macros with more up to date versions
from GCC's longlong.h. Note, GCC's version is licensed under
LGPLv2.1+.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>