Fix building for the x32 target without asm modules.

* mpi/generic/mpi-asm-defs.h: Use a fixed value for the x32 ABI.
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See commit fd6721c235a5bdcb332c8eb708fbd4f96e52e824 for details.
mpi: Use BSD syntax for x86_64-apple-darwin

* mpi/config.links: Add case for x86_64-apple-darwin.
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Suggested by gniibe on 2014-04-24.
Fix bug inhibiting the use of the sentinel attribute.

* src/gcrypt.h.in: Fix typo in macro.
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Reported-by: Rafaël Carré <funman@videolan.org>
mpi: Fix regression for powerpc-apple-darwin detection.

* mpi/config.links: Add separate entry for powerpc-apple-darwin.
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GnuPG-bug-id: 1616