jit.c uses the regcache in a slightly different way, the regcache dosen't
write through to target, but it has read and write methods.  If I apply
regcache in record-full.c, it has the similar use pattern.  This patch
adds a new class detached_regcache, a register buffer, but can be
read and written.

Since jit.c doesn't want to write registers through to target, it uses
regcache as a readonly regcache (because only readonly regcache
disconnects from the target), but it adds a hole in regcache
(raw_set_cached_value) in order to modify a readonly regcache.  This patch
fixes this hole completely.

regcache inherits detached_regcache, and detached_regcache inherits
readable_regcache.  The ideal design is that both detached_regcache and
readable_regcache inherit reg_buffer, and regcache inherit
detached_regcache and regcache_read (virtual inheritance).  I concern
about the performance overhead of virtual inheritance, so I don't do it in
the patch.

gdb:

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

	* jit.c (struct jit_unwind_private) <regcache>: Change its type to
	 reg_buffer_rw *.
	(jit_unwind_reg_set_impl): Call raw_supply.
	(jit_frame_sniffer): Use reg_buffer_rw.
	* record-full.c (record_full_core_regbuf): Change its type.
	(record_full_core_open_1): Use reg_buffer_rw.
	(record_full_close): Likewise.
	(record_full_core_fetch_registers): Use regcache->raw_supply.
	(record_full_core_store_registers): Likewise.
	* regcache.c (regcache::get_register_status): Move it to
	reg_buffer.
	(regcache_raw_set_cached_value): Remove.
	(regcache::raw_set_cached_value): Remove.
	(regcache::raw_write): Call raw_supply.
	(regcache::raw_supply): Move it to reg_buffer_rw.
	* regcache.h (regcache_raw_set_cached_value): Remove.
	(reg_buffer_rw): New class.