Move declarations of debug_threads and using_threads to header
    
    Instead of declaring debug_threads and using_threads in several
    .c files, declare them in debug.h.
    
    gdb/gdbserver/ChangeLog:
    
    2019-09-20  Christian Biesinger  <cbiesinger@google.com>
    
    	* debug.c (debug_threads): Remove comment in favor of the header.
    	* debug.h (using_threads): Add declaration.
    	(debug_threads): Add comment.
    	* linux-aarch64-low.c: Include debug.h and remove declaration of
    	debug_threads.
    	* nto-low.c: Likewise.
    	* remote-utils.c: Likewise.
    	* thread-db.c: Likewise.