mpi/longlong.h: fix missing macro parameter parentheses

* mpi/longlong.h [__alpha] (umul_ppmm): Add parentheses around
used parameters.
[__i370__] (sdiv_qrnnd): Ditto.
[__mips__] (umul_ppmm): Ditto.
[__vax__] (sdiv_qrnnd): Ditto.

Noticed issue after wrong results on mips64 with new mpi/ec code.

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