Remove regcache::m_readonly_p

Now, m_readonly_p is always false, so we can remove it, and regcache no
longer includes pseudo registers.  Some regcache methods are lift up to
its parent class, like reg_buffer or detached_regcache.

gdb:

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

	* regcache.c (regcache::regcache): Update.
	(regcache::invalidate): Move it to detached_regcache::invalidate.
	(get_thread_arch_aspace_regcache): Update.
	(regcache::raw_update): Update.
	(regcache::cooked_read): Remove some code.
	(regcache::cooked_read_value): Likewise.
	(regcache::raw_write): Remove assert on m_readonly_p.
	(regcache::raw_supply_integer): Move it to
	detached_regcache::raw_supply_integer.
	(regcache::raw_supply_zeroed): Likewise.
	* regcache.h (detached_regcache) <raw_supply_integer>: New
	declaration.
	<raw_supply_zeroed, invalidate>: Likewise.
	(regcache) <raw_supply_integer, raw_supply_zeroed>: Removed.
	<invalidate>: Likewise.
	<m_readonly_p>: Removed.