MIPS: fix local_{add,sub}_return on MIPS64

Use "daddu/dsubu" for long int on MIPS64 instead of "addu/subu"

Fixes: 7232311e ("local_t: mips extension")
Signed-off-by: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>