Add branch trace information to struct thread_info.

Add branch trace information to struct thread_info.

Add functions to enable, disable, clear, and fetch a thread's branch trace.

gdb/
	* target.h: Include btrace.h.
	(struct target_ops) <to_supports_btrace, to_enable_btrace,
	to_disable_btrace, to_teardown_btrace, to_read_btrace>: New.
	* target.c (target_supports_btrace): New function.
	(target_enable_btrace): New function.
	(target_disable_btrace): New function.
	(target_teardown_btrace): New function.
	(target_read_btrace): New function.
	* btrace.h: New file.
	* btrace.c: New file.
	* Makefile.in: Add btrace.c.
	* gdbthread.h: Include btrace.h.
	(struct thread_info): Add btrace field.
	* thread.c: Include btrace.h.
	(clear_thread_inferior_resources): Call target_teardown_btrace.
	* common/btrace-common.h: New file.