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
ecc: Add cofactor to domain parameters.

* src/ec-context.h (mpi_ec_ctx_s): Add cofactor 'h'.
* cipher/ecc-common.h (elliptic_curve_t): Add cofactor 'h'.
(_gcry_ecc_update_curve_param): New API adding cofactor.

* cipher/ecc-curves.c (ecc_domain_parms_t): Add cofactor 'h'.
(ecc_domain_parms_t domain_parms): Add cofactors.
(_gcry_ecc_fill_in_curve, _gcry_ecc_update_curve_param)
(_gcry_ecc_get_curve, _gcry_mpi_ec_new, _gcry_ecc_get_param_sexp)
(_gcry_ecc_get_mpi): Handle cofactor.
* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_genkey): Likewise.
* cipher/ecc-misc.c (_gcry_ecc_curve_free)
(_gcry_ecc_curve_copy): Likewise.
* cipher/ecc.c (nist_generate_key, ecc_generate)
(ecc_check_secret_key, ecc_sign, ecc_verify, ecc_encrypt_raw)
(ecc_decrypt_raw, _gcry_pk_ecc_get_sexp, _gcry_pubkey_spec_ecc):
Likewise.
(compute_keygrip): Handle cofactor, but skip it for its computation.
* mpi/ec.c (ec_deinit): Likewise.
* tests/t-mpi-point.c (context_param): Likewise.
(test_curve): Add cofactors.
* tests/curves.c (sample_key_1, sample_key_2): Add cofactors.
* tests/keygrip.c (key_grips): Add cofactors.
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We keep compatibility of compute_keygrip in cipher/ecc.c.