runtime: add support for os/signal

This adds support for enabling and listening to signals on Linux and
MacOS.

src/os/signal/signal.go

@@ -1,14 +0,0 @@
-// Copyright 2012 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package signal
-
-import (
-	"os"
-)
-
-// Just stubbing the functions for now since signal handling is not yet implemented in tinygo
-func Reset(sig ...os.Signal)                      {}
-func Ignore(sig ...os.Signal)                     {}
-func Notify(c chan<- os.Signal, sig ...os.Signal) {}