Move register_dump to regcache-dump.c

gdb:

2017-11-30  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (SFILES): Add regcache-dump.c
	(COMMON_OBS): Add regcache-dump.o.
	* regcache-dump.c: New file.
	* regcache.c: Move register_dump to regcache-dump.c.
	(maintenance_print_registers): Likewise.
	(maintenance_print_raw_registers): Likewise.
	(maintenance_print_cooked_registers): Likewise.
	(maintenance_print_register_groups): Likewise.
	(maintenance_print_remote_registers): Likewise.
	(_initialize_regcache): Likewise.
	* regcache.h (register_dump): Moved from regcache.c.