MIPS N64 ABI passes arguments in registers R4-R11, return value in R2.
R16-R23, R28, R30 and F24-F31 are callee-save. gcc PIC code expects
to be called with indirect call through R25.

Change-Id: I24f582b4b58e1891ba9fd606509990f95cca8051
Reviewed-on: https://go-review.googlesource.com/19805Reviewed-by: Minux Ma <minux@golang.org>