Use MBEDTLS_HAVE_NEON_INTRINSICS instead of __ARM_NEON

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/sha512.c b/library/sha512.c
index 23e8745..0e99914 100644
--- a/library/sha512.c
+++ b/library/sha512.c
@@ -31,7 +31,7 @@
  * By defining the macros ourselves we gain access to those declarations without
  * requiring -march on the command line.
  *
- * `arm_neon.h` could be included by any header file, so we put these defines
+ * `arm_neon.h` is included by common.h, so we put these defines
  * at the top of this file, before any includes.
  */
 #define __ARM_FEATURE_SHA512 1
@@ -60,9 +60,7 @@
 #  if defined(MBEDTLS_SHA512_USE_A64_CRYPTO_IF_PRESENT) || \
     defined(MBEDTLS_SHA512_USE_A64_CRYPTO_ONLY)
 /* *INDENT-OFF* */
-#   ifdef __ARM_NEON
-#       include <arm_neon.h>
-#   else
+#   if !defined(MBEDTLS_HAVE_NEON_INTRINSICS)
 #       error "Target does not support NEON instructions"
 #   endif
 /*