syscall: implementing some mingw syscalls required by os package

R=rsc
CC=golang-dev
https://golang.org/cl/770041
diff --git a/src/pkg/syscall/syscall_unix.go b/src/pkg/syscall/syscall_unix.go
index a32c275..c547ba5 100644
--- a/src/pkg/syscall/syscall_unix.go
+++ b/src/pkg/syscall/syscall_unix.go
@@ -4,6 +4,12 @@
 
 package syscall
 
+var (
+	Stdin  = 0
+	Stdout = 1
+	Stderr = 2
+)
+
 func Errstr(errno int) string {
 	if errno < 0 || errno >= int(len(errors)) {
 		return "error " + str(errno)