Add return address collection for tracepoints.

	Add return address collection for tracepoints.
	* tracepoint.c (encode_actions_1): Add case for $_ret.
	(validate_actionline): Check for $_ret.
	(trace_dump_actions): Ditto.
	* ax-gdb.h (gen_trace_for_return_address): Declare.
	* ax-gdb.c: Include arch-utils.h.
	(gen_trace_for_return_address): New function.
	(agent_command): Add return address special case.
	* amd64-tdep.c: Include ax.h and ax-gdb.h.
	(amd64_gen_return_address): New function.
	(amd64_init_abi): Call it.
	* i386-tdep.c: Include ax.h and ax-gdb.h.
	(i386_gen_return_address): New function.
	(i386_init_abi): Call it.
	* arch-utils.h (default_gen_return_address): Declare.
	* arch-utils.c (default_gen_return_address): New function.
	* gdbarch.sh (gen_return_address): New method.
	* gdbarch.h, gdbarch.c: Regenerate.

	* gdb.texinfo (Tracepoint Action Lists): Document $_ret.

	* gdb.trace/collection.exp: Test collection of $_ret.