Added DiscriminatedPtr support to arm64

Summary: Closes https://github.com/facebook/folly/pull/429

Reviewed By: mzlee

Differential Revision: D3649806

Pulled By: Orvid

fbshipit-source-id: 77288e0a1aedf15a18f558e18fbc1dc3a56e5fd6

folly/DiscriminatedPtr.h

@@ -33,8 +33,8 @@
 #include <folly/Portability.h>
 #include <folly/detail/DiscriminatedPtrDetail.h>
 
-#if !FOLLY_X64 && !FOLLY_PPC64
-# error "DiscriminatedPtr is x64 and ppc64 specific code."
+#if !FOLLY_X64 && !FOLLY_A64 && !FOLLY_PPC64
+# error "DiscriminatedPtr is x64, arm64 and ppc64 specific code."
 #endif
 
 namespace folly {