mpi: Fix for buidling for MIPS64 with Clang

* mpi/longlong.h [MIPS64][__clang__]: Use the C version like we
already do for 32 bit MIPS.

(cherry picked from commit e7ae0ae243c8978a67c802169183187d88557be8)

• GnuPG-bug-id: T3915
• Signed-off-by: Werner Koch <wk@gnupg.org>