crypto/bn/*: x86[_64] division instruction doesn't handle constants, change...

crypto/bn/*: x86[_64] division instruction doesn't handle constants, change constraint from 'g' to 'r'.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(cherry picked from commit 68b4a6e9)