Add support for gdb remote $X stop notification.

debugserver now returns $X09 as the immediate response to
a $k kill process request rather than $W09.

ProcessGDBRemote now properly handles X as indication of
a process exit state.

The @debugserver_test and @lldb_test for $k now properly expects
an X notification (signal-caused exit) after killing a just-attached
inferior that was still in the stopped state.