mpi: Distribute asm definitions for amd64

* mpi/amd64/distfiles: Add mpi-asm-defs.h.

The problem exhibits itself only on Windows64 where
sizeof(long) == sizeof(int).
For other platforms the definition from
generic/mpi-asm-defs.h works also for amd64.

• GnuPG-bug-id: T3184
• Signed-off-by: Werner Koch <wk@gnupg.org>