Add dlmalloc_usable_size for 32 bit arches.

Bug: 17337831

(cherry picked from commit 1c365cb494d8cab1b639ecef21514425647e606b)

Change-Id: Ice100e42eeac4b9c3b77737a546ec33174adcb2f
diff --git a/libc/bionic/dlmalloc.h b/libc/bionic/dlmalloc.h
index 46efa91..ef78814 100644
--- a/libc/bionic/dlmalloc.h
+++ b/libc/bionic/dlmalloc.h
@@ -34,6 +34,13 @@
 
 #define malloc_getpagesize getpagesize()
 
+/* dlmalloc_usable_size was exposed in the NDK, so change the name
+ * of the function on 32 bit architectures.
+ */
+#if !defined(__LP64__)
+#define dlmalloc_usable_size dlmalloc_usable_size_real
+#endif
+
 /* Export two symbols used by the VM. */
 __BEGIN_DECLS
 int dlmalloc_trim(size_t) __LIBC_ABI_PUBLIC__;