contrib: gcrypt: set ac_cv_sys_symbol_underscore on all x86_64 Darwin targets
This should fix link issues like:
Undefined symbols for architecture x86_64:
"__gcry_mpih_add_n", referenced from:
__gcry_mpi_add in libgcrypt.a(mpi-add.o)
_ec_addm_25519 in libgcrypt.a(ec.o)
_ec_subm_25519 in libgcrypt.a(ec.o)
_ec_mulm_25519 in libgcrypt.a(ec.o)
_ec_addm_448 in libgcrypt.a(ec.o)
_ec_subm_448 in libgcrypt.a(ec.o)
_ec_mulm_448 in libgcrypt.a(ec.o)
...
(maybe you meant: __gcry_mpih_add_n_cond)
The symbol names depend on C_SYMBOL_NAME
which depends on ac_cv_sys_symbol_underscore
being set or not.
See https://code.videolan.org/videolan/vlc-unity/-/jobs/1849696