Introduce common_get_thread_regcache

This introduces common_get_thread_regcache so that we can simplify
nat/linux-btrace.c.  A better long term solution would be unify the
regcache code, but this is sufficient for now.

gdb/
2014-06-18  Tom Tromey  <tromey@redhat.com>
	    Gary Benson  <gbenson@redhat.com>

	* regcache.h (common_get_thread_regcache): Declare.
	* regcache.c (common_get_thread_regcache): New function.
	* nat/linux-btrace.h (common_get_thread_regcache): Declare.
	* nat/linux-btrace.c (perf_event_read_bts): Use
	common_get_thread_regcache.

gdb/gdbserver/
2014-06-18  Tom Tromey  <tromey@redhat.com>

	* regcache.c (common_get_thread_regcache): New function.
Finally remove GDBSERVER (mostly) from linux-btrace.c

This applies the usual GDBSERVER treatment to linux-btrace.c.
Now it is used only to find the gnulib header.

gdb/
2014-06-18  Tom Tromey  <tromey@redhat.com>
	    Gary Benson  <gbenson@redhat.com>

	* nat/linux-btrace.c: Don't include defs.h or server.h.
	* nat/linux-btrace.h (struct target_ops): Declare.