gmp: disable assembly for MIPS r6 ISA

It's still not supported/implemented, fixes:
http://autobuild.buildroot.net/results/d06/d0686b3e7b8884f93034d4ea4b79fa8590ffa221/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>