Enable 64 bit atomics on ARM64.

Previously they were disabled due to performance concerns on 32bit
arm, where 64bit atomics are often implemented via kernel traps.

Author: Roman Shaposhnik
Discussion: http://postgr.es/m/CA+ULb+uErkFuXUCCXWHYvnV5KnAyjGUzzRcPA-M0cgO+Hm4RSA@mail.gmail.com

src/include/port/atomics/arch-arm.h

@@ -18,8 +18,9 @@
 #endif
 
 /*
- * 64 bit atomics on arm are implemented using kernel fallbacks and might be
- * slow, so disable entirely for now.
- * XXX: We might want to change that at some point for AARCH64
+ * 64 bit atomics on ARM32 are implemented using kernel fallbacks and thus
+ * might be slow, so disable entirely. On ARM64 that problem doesn't exist.
  */
+#if !defined(__aarch64__) && !defined(__aarch64)
 #define PG_DISABLE_64_BIT_ATOMICS
+#endif  /* __aarch64__ || __aarch64 */