Use tag dispatch regcache ctor in regcache_dup

Use tag dispatch regcache ctor in regcache_dup

This patch adds a tag dispatch ctor to create read-only regcache from
a write-through regcache, also this patch deletes copy ctor and
assignment operator.

gdb:

2017-04-28  Yao Qi  <yao.qi@linaro.org>
	    Pedro Alves  <palves@redhat.com>

	* regcache.c (regcache::regcache): New tag dispatch ctor.
	(do_cooked_read): Moved above.
	(regcache_dup): Use the tag dispatch ctor..
	* regcache.h (regcache): Declare ctor, delete copy ctor and
	assignment operator, remove friend regcache_dup.