cmd/gc: fix undefined behaviour warnings in mparith3.c

Update #8527

Fixes two warnings:

src/cmd/gc/mparith3.c:255:10: runtime error: shift exponent 52 is too large for 32-bit type 'int'
src/cmd/gc/mparith3.c:254:14: runtime error: shift exponent 52 is too large for 32-bit type 'int'

LGTM=rsc
R=r, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/134940044