Make libgcrypt build with Clang on i386

* cipher/longlong.h [__i386__] (add_ssaaaa, sub_ddmmss)
(umul_ppmm, udiv_qrnnd): Do not cast asm output to USItype.

Clang defines GNUC even when it's not GCC compatible. As result Clang
enables GCC-only assembly code in mpi/longlong.h and fails to build.

However, since changes to make libgcrypt build with Clang are smallish, and
changes do not cause problems with GCC, patch just does them.

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