build, cmd/5l: actually report failures for -linkmode tests

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7938046

src/cmd/5l/obj.c

@@ -136,9 +136,11 @@ main(int argc, char *argv[])
 	if(argc != 1)
 		usage();
 
-	if(linkmode != LinkInternal) {
+	if(linkmode == LinkExternal) {
 		diag("only -linkmode=internal is supported");
 		errorexit();
+	} else if(linkmode == LinkAuto) {
+		linkmode = LinkInternal;
 	}
 
 	libinit();

src/run.bash

@@ -79,6 +79,7 @@ go run $GOROOT/test/run.go - .
 
 [ "$CGO_ENABLED" != 1 ] ||
 (xcd ../misc/cgo/test
+set -e
 go test -ldflags '-linkmode=auto'
 go test -ldflags '-linkmode=internal'
 case "$GOHOSTOS-$GOARCH" in