MIPS: fix a latent bug on initialize $gp

If bal is 8 bytes aligned, the _gp will not be 8 bytes aligned.
then the following ld insntrustion generates a Adel exception.
So here make _gp be always aligned in 8 bytes.

Signed-off-by: Zhi-zhou Zhang <zhizhou.zh@gmail.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>