decode: fix NEON inclusion (#714)

The macro that checks for NEON support should be __ARM_NEON, not
__ARM_NEON__. [1]

AArch64 compilers define __ARM_NEON but not __ARM_NEON__.
AArch32 compilers currently seem to define both, but could be within their
rights to drop __ARM_NEON__ in future versions.

This change moves the check into the common/platform.h file, checks for
both forms, and sets BROTLI_TARGET_NEON if NEON support is available.

[1] Section 6.5.4 of the ARM C Language Extensions.
    (At the time of writing, the latest version was Release 2.1.)
diff --git a/c/common/platform.h b/c/common/platform.h
index 3945a27..93db089 100755
--- a/c/common/platform.h
+++ b/c/common/platform.h
@@ -203,6 +203,10 @@
 #endif  /* ARMv8 */
+#if defined(__ARM_NEON__) || defined(__ARM_NEON)
 #if defined(__i386) || defined(_M_IX86)
 #define BROTLI_TARGET_X86
diff --git a/c/dec/decode.c b/c/dec/decode.c
index 1fde329..8f72e85 100644
--- a/c/dec/decode.c
+++ b/c/dec/decode.c
@@ -6,7 +6,7 @@
 #include <brotli/decode.h>
-#if defined(__ARM_NEON__)
+#if defined(BROTLI_TARGET_NEON)
 #include <arm_neon.h>
@@ -167,7 +167,7 @@
 static BROTLI_INLINE void memmove16(uint8_t* dst, uint8_t* src) {
-#if defined(__ARM_NEON__)
+#if defined(BROTLI_TARGET_NEON)
   vst1q_u8(dst, vld1q_u8(src));
   uint32_t buffer[4];