avcodec/videotoolboxenc: #define TARGET_CPU_ARM64 to 0 if not provided by the SDK

Fixes build failure on older SDKs without it.

Fixes #9242

(cherry picked from commit f9eb440e788c08c489bb1a6d24dbbe34f4597f1c)
Signed-off-by: Zane van Iperen <zane@zanevaniperen.com>

libavcodec/videotoolboxenc.c

@@ -49,6 +49,10 @@ enum { kCVPixelFormatType_420YpCbCr10BiPlanarFullRange = 'xf20' };
 enum { kCVPixelFormatType_420YpCbCr10BiPlanarVideoRange = 'x420' };
 #endif
 
+#ifndef TARGET_CPU_ARM64
+#   define TARGET_CPU_ARM64 0
+#endif
+
 typedef OSStatus (*getParameterSetAtIndex)(CMFormatDescriptionRef videoDesc,
                                            size_t parameterSetIndex,
                                            const uint8_t **parameterSetPointerOut,