Don't read agent symbols when disabled

Don't read agent symbols when disabled

This avoids unnecessary work, and becomes important with the patch in
https://sourceware.org/ml/gdb-patches/2019-10/msg01143.html

gdb/ChangeLog:

2019-10-31  Christian Biesinger  <cbiesinger@google.com>

	* agent.c (set_can_use_agent): When the setting is turned on,
	look up agent symbols if we don't have them yet.
	(agent_new_objfile): Don't look up agent symbols when the agent
	setting is off.

Change-Id: I6523a5640c95d38299998050a6c620e51096e8ed