cmd/go: import runtime/cgo into darwin/arm64 tests

Until cl/12721 and cl/12574, all standard library tests included
runtime/cgo on darwin/arm64 by virtue of package os including it. Now
that is no longer true, runtime/cgo needs to be added by the go tool
just as it is for darwin/arm. (This installs the Mach exception
handler used to properly handle EXC_BAD_ACCESS.)

Fixes #11901

Change-Id: I991525f46eca5b0750b93595579ebc0ff10e47eb
Reviewed-on: https://go-review.googlesource.com/12723Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/test.go

@@ -804,8 +804,10 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		recompileForTest(pmain, p, ptest, testDir)
 	}
 
-	if buildContext.GOOS == "darwin" && buildContext.GOARCH == "arm" {
-		t.NeedCgo = true
+	if buildContext.GOOS == "darwin" {
+		if buildContext.GOARCH == "arm" || buildContext.GOARCH == "arm64" {
+			t.NeedCgo = true
+		}
 	}
 
 	for _, cp := range pmain.imports {