Simplify regcache_dup

Simplify regcache_dup

regcache_dup, in fact, is to create a readonly regcache from a
non-readonly regcache.  This patch adds an assert that src is not
readonly.

gdb:

2017-04-28  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache_dup): Assert !src->m_readonly_p and
	call method save instead of regcache_cpy.
	* regcache.h (struct regcache): Make regcache_dup a friend.