src/os: export correct values for os.DevNull for each OS

This commit introduces `os.DevNull` exports for all supported OS
targets.

src/os/file_unix.go

@@ -1,6 +1,9 @@
 //go:build darwin || (linux && !baremetal)
 // +build darwin linux,!baremetal
 
+// target wasi sets GOOS=linux and thus the +linux build tag,
+// even though it doesn't show up in "tinygo info target -wasi"
+
 // Portions copyright 2009 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.
@@ -12,6 +15,8 @@ import (
 	"syscall"
 )
 
+const DevNull = "/dev/null"
+
 type syscallFd = int
 
 // fixLongPath is a noop on non-Windows platforms.