Add a pages_purge function to wrap madvise(JEMALLOC_MADV_PURGE) calls

This will be used to implement the feature on mingw, which doesn't have
madvise.

src/arena.c

@@ -676,8 +676,8 @@ arena_chunk_purge(arena_t *arena, arena_chunk_t *chunk)
 		if (config_debug)
 			ndirty -= npages;
 
-		madvise((void *)((uintptr_t)chunk + (pageind << LG_PAGE)),
-		    (npages << LG_PAGE), JEMALLOC_MADV_PURGE);
+		pages_purge((void *)((uintptr_t)chunk + (pageind << LG_PAGE)),
+		    (npages << LG_PAGE));
 		if (config_stats)
 			nmadvise++;
 	}

src/chunk.c

@@ -171,7 +171,7 @@ chunk_record(void *chunk, size_t size)
 {
 	extent_node_t *xnode, *node, *prev, key;
 
-	madvise(chunk, size, JEMALLOC_MADV_PURGE);
+	pages_purge(chunk, size);
 
 	xnode = NULL;
 	malloc_mutex_lock(&chunks_mtx);

src/chunk_mmap.c

@@ -72,6 +72,20 @@ pages_unmap(void *addr, size_t size)
 	}
 }
 
+void
+pages_purge(void *addr, size_t length)
+{
+
+#ifdef JEMALLOC_PURGE_MADVISE_DONTNEED
+#  define JEMALLOC_MADV_PURGE MADV_DONTNEED
+#elif defined(JEMALLOC_PURGE_MADVISE_FREE)
+#  define JEMALLOC_MADV_PURGE MADV_FREE
+#else
+#  error "No method defined for purging unused dirty pages."
+#endif
+	madvise(addr, length, JEMALLOC_MADV_PURGE);
+}
+
 static void *
 chunk_alloc_mmap_slow(size_t size, size_t alignment, bool unaligned)
 {