Add the gdb remote target operations for branch tracing.

Add the gdb remote target operations for branch tracing.
We define the following packets:

  Qbtrace:bts         enable branch tracing for the current thread
                      returns "OK" or "Enn"

  Qbtrace:off         disable branch tracing for the current thread
                      returns "OK" or "Enn"

  qXfer:btrace:read   read the full branch trace data for the current thread

gdb/
	* target.h (enum target_object): Add TARGET_OBJECT_BTRACE.
	* remote.c: Include btrace.h.
	(struct btrace_target_info): New struct.
	(remote_supports_btrace): New function.
	(send_Qbtrace): New function.
	(remote_enable_btrace): New function.
	(remote_disable_btrace): New function.
	(remote_teardown_btrace): New function.
	(remote_read_btrace): New function.
	(init_remote_ops): Add btrace ops.
	(enum <unnamed>): Add btrace packets.
	(struct protocol_feature remote_protocol_features[]): Add btrace packets.
	(_initialize_remote): Add packet configuration for branch tracing.

gdbserver/
	* target.h (struct target_ops): Add btrace ops.
	(target_supports_btrace): New macro.
	(target_enable_btrace): New macro.
	(target_disable_btrace): New macro.
	(target_read_btrace): New macro.
	* gdbthread.h (struct thread_info): Add btrace field.
	* server.c: Include btrace-common.h.
	(handle_btrace_general_set): New function.
	(handle_btrace_enable): New function.
	(handle_btrace_disable): New function.
	(handle_general_set): Call handle_btrace_general_set.
	(handle_qxfer_btrace): New function.
	(struct qxfer qxfer_packets[]): Add btrace entry.
	* inferiors.c (remove_thread): Disable btrace.
	* linux-low: Include linux-btrace.h.
	(linux_low_enable_btrace): New function.
	(linux_low_read_btrace): New function.
	(linux_target_ops): Add btrace ops.
	* configure.srv (i[34567]86-*-linux*): Add linux-btrace.o.
	Add srv_linux_btrace=yes.
	(x86_64-*-linux*): Add linux-btrace.o.
	Add srv_linux_btrace=yes.
	* configure.ac: Define HAVE_LINUX_BTRACE.
	* config.in: Regenerated.
	* configure: Regenerated.