gdb: turn arc_debug into a bool

gdb: turn arc_debug into a bool

Shahab suggested we get rid of the verbosity level for the ARC debug
logging [1].  This patch does that, before doing any other change.

gdb/ChangeLog:

	* arc-tdep.h (arc_debug): Change type to bool.
	* arc-tdep.c (arc_debug): Change type to bool.
	(arc_analyze_prologue): Adjust.
	(_initialize_arc_tdep): Use add_setshow_boolean_cmd.
	* arc-linux-nat.c (ps_get_thread_area): Adjust.

[1] https://sourceware.org/pipermail/gdb-patches/2021-January/175075.html

Change-Id: I16688bd42ed8978ae1acf57012c8d41a943044a5