Use MOVW, instead of MOVV, to pass an int32 arg. Also no need to
restore arg registers.

Fix big-endian MIPS64 build.

Change-Id: Ib43c71075c988153e5e5c5c6e7297b3fee28652a
Reviewed-on: https://go-review.googlesource.com/28830Reviewed-by: Minux Ma <minux@golang.org>