runtime: include stderr when objdump fails

If objdump panics, we want the panic included in the test log.

Change-Id: I6a6a636c13c5ba08acaa1c6c8714541a8e91542b
Reviewed-on: https://go-review.googlesource.com/c/go/+/704338
Reviewed-by: Florian Lehner <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

src/runtime/unsafepoint_test.go

@@ -43,7 +43,7 @@ func TestUnsafePoint(t *testing.T) {
 	cmd := exec.Command(testenv.GoToolPath(t), "tool", "objdump", "-s", "setGlobalPointer", os.Args[0])
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		t.Fatalf("can't objdump %v", err)
+		t.Fatalf("can't objdump %v:\n%s", err, out)
 	}
 	lines := strings.Split(string(out), "\n")[1:]