runtime: syscall to return both AX and DX for windows/386

Fixes #2181.

R=golang-dev, jp
CC=golang-dev
https://golang.org/cl/5000042
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 6feedcb..8753842 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -307,7 +307,8 @@
 	void	(*fn)(void*);
 	uintptr	n;	// number of parameters
 	void*	args;	// parameters
-	uintptr	r;	// return value
+	uintptr	r1;	// return values
+	uintptr	r2;
 	uintptr	err;	// error number
 };