Remove regcache_save and regcache_cpy

Remove regcache_save and regcache_cpy

... instead we start to use regcache methods save and restore.  It is
quite straightforward to replace regcache_save with regcache->save.

regcache_cpy has some asserts, some of them not necessary, like

 gdb_assert (src != dst);

because we already assert !m_readonly_p and src->m_readonly_p, so
src isn't dst.  Some of the asserts are moved to ::restore.

gdb:

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

	* frame.c (frame_save_as_regcache): Use regcache method save.
	(frame_pop): Use regcache method restore.
	* infrun.c (restore_infcall_suspend_state): Likewise.
	* linux-fork.c (fork_load_infrun_state): Likewise.
	* ppc-linux-tdep.c (ppu2spu_sniffer): User regcache method
	save.
	* regcache.c (regcache_save): Remove.
	(regcache::restore): More asserts.
	(regcache_cpy): Remove.
	* regcache.h (regcache_save): Remove the declaration.
	(regcache::restore): Move from private to public.
	Remove the friend declaration of regcache_cpy.
	(regcache_cpy): Remove declaration.