runtime: add windows callback tests

Just a copy of cgo callback tests from misc/cgo/test.

R=rsc
CC=golang-dev, hectorchu
https://golang.org/cl/5331062

src/pkg/runtime/export_test.go

@@ -18,6 +18,8 @@ var F64toint = f64toint
 
 func entersyscall()
 func exitsyscall()
+func golockedOSThread() bool
 
 var Entersyscall = entersyscall
 var Exitsyscall = exitsyscall
+var LockedOSThread = golockedOSThread