misc/cgo/test: test recursive internal OS thread lock

This would have failed with CL 11663043.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/11480045

misc/cgo/test/issue3775.go

@@ -15,6 +15,14 @@ import (
 	"testing"
 )
 
+func init() {
+	// Same as test3775 but run during init so that
+	// there are two levels of internal runtime lock
+	// (1 for init, 1 for cgo).
+	// This would have been broken by CL 11663043.
+	C.lockOSThreadC()
+}
+
 func test3775(t *testing.T) {
 	// Used to panic because of the UnlockOSThread below.
 	C.lockOSThreadC()