Introduce common-regcache.h

Introduce common-regcache.h

This introduces common-regcache.h.  This contains two functions that
allow nat/linux-btrace.c to be simplified.  A better long term
solution would be unify the regcache code, but this is sufficient for
now.

gdb/ChangeLog:

	* common/common-regcache.h: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add common/common-regcache.h.
	* regcache.h: Include common-regcache.h.
	(regcache_read_pc): Don't declare.
	* regcache.c (get_thread_regcache_for_ptid): New function.
	* nat/linux-btrace.c: Don't include regcache.h.
	Include common-regcache.h.
	(perf_event_read_bts): Use get_thread_regcache_for_ptid.

gdb/gdbserver/ChangeLog:

	* regcache.h: Include common-regcache.h.
	(regcache_read_pc): Don't declare.
	* regcache.c (get_thread_regcache_for_ptid): New function.