Remove extra lang item, exchange_free; use box_free instead.

Trans used to insert code equivalent to box_free in a wrapper around
exchange_free, and that code is now removed from trans.

src/liballoc/heap.rs

@@ -144,6 +144,7 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 }
 
 #[cfg(not(test))]
+#[cfg(stage0)]
 #[lang = "exchange_free"]
 #[inline]
 unsafe fn exchange_free(ptr: *mut u8, old_size: usize, align: usize) {