btrace: store raw btrace data

btrace: store raw btrace data

Store the raw branch trace data that has been read from the target.

This data can be used for maintenance commands as well as for generating
a core file for the "record save" command.

gdb/
	* btrace.c (btrace_fetch): Append the new trace data.
	(btrace_clear): Clear the stored trace data.
	* btrace.h (btrace_thread_info) <data>: New.
	* common/btrace-common.h (btrace_data_clear)
	(btrace_data_append): New.
	* common/btrace-common.c (btrace_data_clear)
	(btrace_data_append): New.