runtime: make CgoMal alloc field void*

This makes it an unsafe.Pointer in Go so the garbage collector
will treat it as a pointer to untyped data, not a pointer to
bytes.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/8286045

src/pkg/runtime/runtime.h

@@ -511,7 +511,7 @@ struct ParFor
 struct CgoMal
 {
 	CgoMal	*next;
-	byte	*alloc;
+	void	*alloc;
 };
 
 /*