Finally remove GDBSERVER (mostly) from agent.c

This reduces the use of the GDBSERVER define in agent.c to deciding
which gnulib header to include.

gdb/
2014-07-16  Tom Tromey  <tromey@redhat.com>
	    Gary Benson  <gbenson@redhat.com>

	* common/agent.c: Don't include defs.h or server.h; update
	includes.
Introduce get_thread_regcache_for_ptid

This introduces get_thread_regcache_for_ptid 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/
2014-07-16  Tom Tromey  <tromey@redhat.com>
	    Gary Benson  <gbenson@redhat.com>

	* common/common-regcache.h: New file.
	* regcache.c: Include the above.
	(get_thread_regcache_for_ptid): New function.
	* nat/linux-btrace.c: Include common-regcache.h.
	(perf_event_read_bts): Use get_thread_regcache_for_ptid.

gdb/gdbserver/
2014-07-16  Tom Tromey  <tromey@redhat.com>
	    Gary Benson  <gbenson@redhat.com>

	* regcache.c: Include common-regcache.h.
	(get_thread_regcache_for_ptid): 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/
2014-07-16  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.