New class reg_buffer

New class reg_buffer

This patch splits regcache, moves part of it to reg_buffer, which is a
buffer to hold register contents, and also their status.  reg_buffer may
or may not hold pseudo registers.  Class reg_buffer has methods supply and
collect.

gdb:

2017-09-22  Yao Qi  <yao.qi@linaro.org>
:
	* regcache.c (regcache_descr):
	(regcache_register_size):
	(get_regcache_aspace):
	(regcache::save):
	(regcache::save):
	(regcache::restore):
	(regcache_register_status):
	(regcache::get_register_status):
	(regcache::invalidate):
	(regcache::raw_update):
	(regcache::raw_read):
	(regcache::cooked_read):
	(regcache::cooked_read_value):
	(regcache_raw_set_cached_value):
	(regcache_raw_supply):
	(regcache::raw_supply):
	(regcache::raw_supply_integer):
	(regcache_raw_collect):
	(regcache::raw_collect):
	* regcache.h (typedef struct cached_reg):
	(public:):
	(public:):
	(protected:):
	(private:):