Move current_regcache to regcache::current_regcache

Move current_regcache to regcache::current_regcache

This patches moves global variable current_regcache to a class regcache
static variable (protected) so that the unit test I add in the following
patch can access it (by means of extending class regcache in unit test).

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (current_regcache): Change it to
	regcache::current_regcache.
	(regcache_observer_target_changed): Update.
	(regcache_thread_ptid_changed): Make it a regcache static
	method.
	(regcache_thread_ptid_changed): Update.
	(class regcache_access): New.
	(current_regcache_test): Update.
	(_initialize_regcache): Update.
	* regcache.h: Include forward_list.
	(regcache): Declare regcache_thread_ptid_changed and declare
	registers_changed_ptid as friend.