integer-gmp: mpn_gcd_1 allocate one Word on stack instead of 'sizeof (__mpz_struct)'

gmp.h defines 'mpn_gcd_1' as following:
    mp_limb_t mpn_gcd_1 (mp_srcptr /* aka 'mp_limb_t *'  */, mp_size_t, mp_limb_t);

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>