libc: Fix typo that broke NDK compatibility.

The function must be named __atomic_cmpxchg, not __android_cmpxchg.
This typo broke existing prebuilt binaries (they couldn't be loaded
at runtime anymore).

Change-Id: I25ca7d18329817f0056e616a0409113269ad7b1f

libc/arch-arm/bionic/atomics_arm.c

@@ -58,7 +58,7 @@
 /* DO NOT INCLUDE <sys/atomics.h> HERE ! */
 
 int
-__android_cmpxchg(int old, int _new, volatile int *ptr)
+__atomic_cmpxchg(int old, int _new, volatile int *ptr)
 {
     /* We must return 0 on success */
     return __sync_val_compare_and_swap(ptr, old, _new) != old;