mpi: allow building with clang on ARM
* mpi/longlong.h [__arm__] (add_ssaaaa, sub_ddmmss, umul_ppmm) (count_leading_zeros): Do not cast assembly output arguments. [__arm__] (umul_ppmm): Remove the extra '%' ahead of assembly comment. [_ARM_ARCH >= 4] (umul_ppmm): Use correct inputs and outputs instead of registers.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>