runtime: add support for os/signal

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

testdata/signal.go

@@ -0,0 +1,42 @@
+package main
+
+// Test POSIX signals.
+// TODO: run `tinygo test os/signal` instead, once CGo errno return values are
+// supported.
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+func main() {
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, syscall.SIGUSR1)
+
+	// Wait for signals to arrive.
+	go func() {
+		for sig := range c {
+			if sig == syscall.SIGUSR1 {
+				println("got expected signal")
+			} else {
+				println("got signal:", sig.String())
+			}
+		}
+	}()
+
+	// Send the signal.
+	syscall.Kill(syscall.Getpid(), syscall.SIGUSR1)
+
+	time.Sleep(time.Millisecond * 100)
+
+	// Stop notifying.
+	// (This is just a smoke test, it's difficult to test the default behavior
+	// in a unit test).
+	signal.Ignore(syscall.SIGUSR1)
+
+	signal.Stop(c)
+
+	println("exiting signal program")
+}

testdata/signal.txt

@@ -0,0 +1,2 @@
+got expected signal
+exiting signal program