gdbserver crash if gdb attaches too fast

With "maint set target-non-stop on", the attach tests occasionally
crash gdbserver.

Basically, gdb attaches with vAttach;PID, and then shortly after reads
the xml target description for that process, to figure out the
process' architecture.  On the gdbserver side, the target description
is only filled in when the first process/thread in the thread group
reports its initial PTRACE_ATTACH SIGSTOP.  So if GDB is fast enough,
it can read the target description _before_ that initial stop, and
then gdbserver dies dereferencing a NULL tdesc pointer.

gdb/gdbserver/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_attach): In non-stop mode, wait for one stop
	before returning.