Class reg_buffer

Class reg_buffer

This patch adds a new class reg_buffer, and regcache inherits it.  Class
reg_buffer is a very simple class, which has the buffer for register
contents and status only.  It doesn't have any methods to set contents and
status, and it is expected that its children classes can inherit it and
add different access methods.

Another reason I keep class reg_buffer so simple is that I think
reg_buffer can be even reused in other classes which need to record the
registers contents and status, like frame cache for example.

gdb:

2018-02-21  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache::regcache): Call reg_buffer ctor.
	(regcache::arch): Move it to reg_buffer::arch.
	(regcache::register_buffer): Likewise.
	(regcache::assert_regnum): Likewise.
	(regcache::num_raw_registers): Likewise.
	* regcache.h (reg_buffer): New class.
	(regcache): Inherit reg_buffer.