mpi/longlong: fix variable shadowing from MIPS umul_ppmm macros
* mpi/longlong.h [__mips__ && W_TIPE_SIZE == 32] (umul_ppmm): Rename temporary variable '_r' to '__r'. [__mips && W_TIPE_SIZE == 64] (umul_ppmm): Ditto.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>