Constify some commands in regcache.c

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* regcache.c (regcache_print, maintenance_print_registers)
	(maintenance_print_raw_registers)
	(maintenance_print_cooked_registers)
	(maintenance_print_register_groups)
	(maintenance_print_remote_registers): Constify.