Simplify regcache_restore

Simplify regcache_restore

This patches removes the 2nd argument of regcache_restore, because it
is only called by regcache_cpy.  In regcache_cpy, if regcache_restore
is called, dst is not readonly, but src is readonly.  So this patch
adds an assert that src is readonly in regcache_restore.
regcache_cook_read read everything from a readonly regcache cache
(src)'s register_buffer, and register status is from ->register_status.

gdb:

2017-04-21  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache_restore): Remove argument 2.  Replace
	argument 3 with regcache.  Get register status from
	src->register_status and get register contents from
	register_buffer (src, regnum).
	(regcache_cpy): Update.