cmd/5g: fix fixedtests/issue4396b.go test failure on arm5
cmd/5g: fix fixedtests/issue4396b.go test failure on arm5

Add missing file that should have been included in CL 6854063 / 5eac1a2d6fc3

R=remyoudompheng, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/6891049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go