aarch64: use '#' for whole line asm comments

Both gnu as and clang treat lines starting with '#' as comments if they
aren't consumed by the C-style preprocessor.
Using '//' does not work with clang since comments are removed before
macro expansion.

libavutil/aarch64/asm.S

@@ -23,7 +23,7 @@
 #ifdef __ELF__
 #   define ELF
 #else
-#   define ELF //
+#   define ELF  #
 #endif
 
 .macro  function name, export=0, align=2