Better backwards compatibility for stat64.

Caused problems for some Google source switching to unified headers
because the legacy headers just used #define instead.

Bug: http://b/73210428
Test: builds
Change-Id: If8a36f284031e57af419903610029a8c34a23d91
diff --git a/libc/Android.bp b/libc/Android.bp
index 13e17fe..f5491f5 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -33,6 +33,7 @@
 
 libc_common_flags = [
     "-D_LIBC=1",
+    "-D__BIONIC_LP32_USE_STAT64",
     "-Wall",
     "-Wextra",
     "-Wunused",