[release-branch.go1.13] cmd/link: ensure cgo cflags do not leak into tvOS test

Running the 'TestBuildForTvOS' test with CGO_CFLAGS set
with certain values would cause the test to fail. all.bash
would fail when CGO_CFLAGS was set to '-mmacosx-version-min=10.10'
because the --macosx-version-min flag is incompatible with tvOS.
The change guards against using an unintended flag in the unit test.

Updates #36846
Updated #35459

Change-Id: Ifc43f3ebfb23d37aabeaac2ea9efae5b877991bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/215957
Run-TryBot: Carlos Amedee <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
(cherry picked from commit ace25f8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/218598

Author: cagedmantis

src/cmd/link/link_test.go

@@ -214,6 +214,7 @@ func TestBuildForTvOS(t *testing.T) {
 		"GOOS=darwin",
 		"GOARCH=arm64",
 		"CC="+strings.Join(CC, " "),
+		"CGO_CFLAGS=", // ensure CGO_CFLAGS does not contain any flags. Issue #35459
 	)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)