cmd/go: don't run TestIssue7573 if cgo not supported

Fixes #12629.

Change-Id: Iee96dc4f806a38f3cd8e065b8d0d5f682bb7e29b
Reviewed-on: https://go-review.googlesource.com/14597Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -1839,6 +1839,9 @@ func TestIssue6480(t *testing.T) {
 
 // cmd/cgo: undefined reference when linking a C-library using gccgo
 func TestIssue7573(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
 	if _, err := exec.LookPath("gccgo"); err != nil {
 		t.Skip("skipping because no gccgo compiler found")
 	}