arm: fix build2, tweak test/nul1.go arm exclusion

R=rsc
CC=golang-dev
https://golang.org/cl/1866044

test/nul1.go

 // [ $GOOS != nacl ] || exit 0  # NaCl runner elides NUL in output
-// [ "$GORUN" != "a" ] || exit 0  # Android runner gets confused by the NUL output 
+// [ "$GORUN" == "" ] || exit 0  # Android runner gets confused by the NUL output 
 // $G $D/$F.go && $L $F.$A && ./$A.out >tmp.go &&
 // errchk $G -e tmp.go
 // rm -f tmp.go