gdbserver: Move remote_debug to a single place

gdbserver: Move remote_debug to a single place

A comment in debug.h (written in 2014) states: "We declare debug format
variables here, and debug_threads but no other debug content variables
(e.g., not remote_debug) because while this file is not currently used by
IPA it may be some day, and IPA may have its own set of debug content
variables".

This has resulted in remote_debug being declared in many .c/.h files
throughout gdbserver.

It would be much simplier to define it one place.  The most logical place to
define it is in debug.h, surrounded by #define guards.  If IPA is changed,
then at that point the variable can be moved elsewhere.

gdb/gdbserver/ChangeLog:

	* debug.c (remote_debug): Add definition.
	* debug.h (remote_debug): Add declaration.
	* hostio.c (remote_debug): Remove declaration.
	* remote-utils.c (struct ui_file): Likewise.
	(remote_debug): Likewise.
	* remote-utils.h (remote_debug): Likewise,
	* server.c (remote_debug): Remove definition.