mpi: Fix ARM assembler in longlong.h.

* mpi/longlong.h [__arm__] (add_ssaaaa, sub_ddmmss): Add __CLOBBER_CC.
[__arm__][__ARM_ARCH <= 3] (umul_ppmm): Add __AND_CLOBBER_CC.

This is a backport of libgcrypt 8aa4f2161 and 3b1cc9e6c.

• Signed-off-by: Marcus Brinkmann <mb@g10code.com>
• GnuPG-bug-id: T3182