Add W64 support for mpi amd64 assembly

acinclude.m4 (GNUPG_SYS_SYMBOL_UNDERSCORE): Set
'ac_cv_sys_symbol_underscore=no' on MingW-W64.
mpi/amd64/func_abi.h: New.
mpi/amd64/mpih-add1.S (_gcry_mpih_add_n): Add FUNC_ENTRY and FUNC_EXIT.
mpi/amd64/mpih-lshift.S (_gcry_mpih_lshift): Ditto.
mpi/amd64/mpih-mul1.S (_gcry_mpih_mul_1): Ditto.
mpi/amd64/mpih-mul2.S (_gcry_mpih_addmul_1): Ditto.
mpi/amd64/mpih-mul3.S (_gcry_mpih_submul_1): Ditto.
mpi/amd64/mpih-rshift.S (_gcry_mpih_rshift): Ditto.
mpi/amd64/mpih-sub1.S (_gcry_mpih_sub_n): Ditto.
mpi/config.links [host=x86_64-*mingw*]: Enable assembly modules.

[host=x86_64-*-*]: Append mpi/amd64/func_abi.h to mpi/asm-syntax.h.

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