Add back a dummy arc4random_stir for compatibility.

The current arc4random implementation stirs itself as needed, but we
need to keep an arc4random_stir symbol around for binary compatibility.

(cherry-pick of 1e010d60397db706cd3d1c4d5701a2bced441aa8.)

Bug: 17291075
Change-Id: Iaf6171c3ec65c39c1868364d5b35ea280e29a363

libc/bionic/ndk_cruft.cpp

@@ -303,4 +303,9 @@ extern "C" char* index(const char* str, int ch) {
   return strchr(str, ch);
 }
 
+// This was removed from BSD.
+extern "C" void arc4random_stir(void) {
+  // The current implementation stirs itself as needed.
+}
+
 #endif