gdb, btrace: forward fetch_registers for unknown threads

gdb, btrace: forward fetch_registers for unknown threads

In the record-btrace target, while replaying, we can only provide the PC
register.  The btrace state is stored in the thread_info.  So, when trying
to determine whether we are currently replaying, GDB calls
find_thread_ptid() to obtain the thread_info.  It also asserts that we do
have a thread_info.

For new threads, libthread-db may fetch registers before the thread is
known to GDB.  In this case, find_thread_ptid() returns nullptr and the
assertion fails.

Forward the fetch_registers request to the target beneath in that case.

gdb/ChangeLog:

2020-03-19  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_target::fetch_registers): Forward
	request if we do not have a thread_info.

gdb/testsuite/ChangeLog:

2020-03-19  Markus Metzger  <markus.t.metzger@intel.com>

	* gdb.btrace/enable-new-thread.c: New test.
	* gdb.btrace/enable-new-thread.exp: New file.