Fix use of NEON on ARMv8
Fix use of NEON on ARMv8

Flag neon_available_ is automatically set to true when __aarch64__ is defined,
but the actual check for neon_available_ required having also HAVE_NEON defined.

Now we check the flag also when only __aarch64__ is defined.

amitdo/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,stweil/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,stweil/tesseract,stweil/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,stweil/tesseract,stweil/tesseract