gdb: move remote_debug to remote.{h,c}

gdb: move remote_debug to remote.{h,c}

remote_debug is currently declared in target.h and defined in top.c.
Move them to remote.h and remote.c.

Include remote.h in remote-sim.c, as it uses remote_debug.

gdb/ChangeLog:

	* target.h (remote_debug): Move to...
	* remote.h (remote_debug): ... here.
	* top.c (remote_debug): Move to...
	* remote.c (remote_debug): ... here.
	* remote-sim.c: Include remote.h.

Change-Id: Iae632d12ff8900b23eee6b2529d6a3cd339a8caa