cmd/go: skip $ORIGIN test on darwin systems

Fixes #7293.

Update #7261

The bsd ld(1) does not understand $ORIGIN and has restrictions on using -rpath when using clang(1), the default compiler on darwin.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/58480045

src/cmd/go/test.bash

@@ -608,7 +608,7 @@ export GOPATH=$d
 mkdir -p $d/src/origin
 echo '
 package origin
-// #cgo LDFLAGS: -Wl,-rpath -Wl,$ORIGIN
+// #cgo !darwin LDFLAGS: -Wl,-rpath -Wl,$ORIGIN
 // void f(void) {}
 import "C"