Add reg_buffer_common

Add reg_buffer_common

A purely virtual class containing functions from gdb/regcache.h

Both the gdb regcache structures and gdbserver regcache inherit
directly from reg_buffer_common. This will allow for common
functions which require the use of a regcache.

gdb/
	* common/common-regcache.h (reg_buffer_common): New structure.
	* regcache.c (reg_buffer::invalidate): Move from detached_regcache.
	(reg_buffer::raw_supply): Likewise.
	(reg_buffer::raw_supply_integer): Likewise.
	(reg_buffer::raw_supply_zeroed): Likewise.
	(reg_buffer::raw_collect): Likewise.
	(reg_buffer::raw_collect_integer): Likewise.
	* regcache.h (reg_buffer::invalidate): Move from detached_regcache.
	(reg_buffer::raw_supply): Likewise.
	(reg_buffer::raw_supply_integer): Likewise.
	(reg_buffer::raw_supply_zeroed): Likewise.
	(reg_buffer::raw_collect): Likewise.
	(reg_buffer::raw_collect_integer): Likewise.

gdbserver/
	* regcache.c (new_register_cache): Use new.
	(free_register_cache): Use delete.
	(register_data): Use const.
	(supply_register): Move body inside regcache.
	(regcache::raw_supply): New override function.
	(collect_register): Move body inside regcache.
	(regcache::raw_collect): New override function.
	(regcache::get_register_status): New override function.
	* regcache.h (struct regcache): Inherit from reg_buffer_common.