Operations on int64 are very stack consuming with 5c.
Fixes netbsd/arm build.

Before: TEXT    runtime.timediv+0(SB),7,$52-16
After:  TEXT    runtime.timediv+0(SB),7,$44-16

The stack usage is unchanged on 386:
        TEXT    runtime.timediv+0(SB),7,$8-16

R=golang-dev, dvyukov, bradfitz
CC=golang-dev
https://golang.org/cl/12182044