x86_64: minor bignum performance boost
x86_64: minor bignum performance boost

Incorporate a few x86_64 assembly routines from the latest GMP,
which provides a small speed boost for operations such as bignum
multiplication and division.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket