The address space is useless to readonly regcache, so this patch removes
the parameter to construct readonly regcache.

address_space was added in regcache by 6c95b8df, but for read-write
regcache.  regcache::aspace is used for various breakpoint/watchpoint
checking, and these regcache are not read-only regcache.

gdb:

2017-11-02  Yao Qi  <yao.qi@linaro.org>

	* frame.c (do_frame_register_read): Remove aspace.
	* jit.c (jit_frame_sniffer): Likwise.
	* ppc-linux-tdep.c (ppu2spu_sniffer): Likewise.
	* regcache.c (regcache::regcache): Pass nullptr.
	(regcache_print): Caller updated.
	* regcache.h (regcache::regcache): Remove one constructor
	parameter aspace.