btrace: async

btrace: async

The record btrace target runs synchronous with GDB.  That is, GDB steps
resumed threads in record btrace's to_wait method.  Without GDB calling
to_wait, nothing happens 'on the target'.

Check for further expected events in to_wait before reporting the current
event and mark record btrace's async event handler in async mode.

gdb/
	* record-btrace.c (record_btrace_maybe_mark_async_event): New.
	(record_btrace_wait): Call record_btrace_maybe_mark_async_event.