move __malloc_replaced to a top-level malloc file

it's not part of the malloc implementation but glue with musl dynamic
linker.

src/malloc/oldmalloc/malloc.c

@@ -20,8 +20,6 @@ static struct {
 	volatile int split_merge_lock[2];
 } mal;
 
-int __malloc_replaced;
-
 /* Synchronization tools */
 
 static inline void lock(volatile int *lk)

src/malloc/replaced.c

@@ -0,0 +1,3 @@
+#include "dynlink.h"
+
+int __malloc_replaced;