Got basic $? (stop reason query) packet working.

Linux native threads now track stop reason info.  Signals are
covered when marked in the stop state.

Right now Linux signal numbers are used directly.  I suspect this
may need to be converted to gdb signal numbers.

lldb no longer hangs when trying to run an exe on llgs now that
$? is answering.  However, llgs shuts down the moment it gets a
query for register info, which it cannot answer yet.

Also, $? does not yet provide all threads in stop reply.  That
needs to be hooked up.

Added method to NativeThreadProtocol to handle translating stop
info exceptions into a gdb remote signal number.  Added enum
for GdbSignal to lldb-enumerations.h in preparation for this.
RNBRemote.cpp has some code that MacOSX will want to implement
to translate Mach exception info to Gdb signals.