include legacy __ARM_NEON__ check for FOLLY_NEON

Summary:
According to https://github.com/pytorch/pytorch/pull/6697, ARM64 clang from the
Android SDK does not define the newer `__ARM_NEON` macro (specified in the ARM
C language extensions 2.1:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ihi0053d/index.html),
but rather, it uses `__ARM_NEON__`.

Portability should check for both until full deprecation?

Reviewed By: nbronson

Differential Revision: D9163276

fbshipit-source-id: db6407f85b21b0ad3e4d564c73ef3ca5d8a16c2f

folly/Portability.h

@@ -302,7 +302,7 @@ constexpr auto kIsBigEndian = !kIsLittleEndian;
   (FOLLY_SSE > major || FOLLY_SSE == major && FOLLY_SSE_MINOR >= minor)
 
 #ifndef FOLLY_NEON
-# if defined(__ARM_NEON)
+# if defined(__ARM_NEON) || defined(__ARM_NEON__)
 #  define FOLLY_NEON 1
 # endif
 #endif