gdb(4): Implement qXfer:threads:read

This streams out an XML document over several GDB packets describing all
threads in the system; their ids, name, and any loosely defined "extra info"
we feel like including. For now, I have included a string version of the run
state, similar to some of the DDB logic to stringify thread state.

The benefit of supporting this in addition to the qfThreadInfo/qsThreadInfo
packing is that in this mode, the host gdb does not ask for every thread's
"qThreadExtraInfo," saving per-thread round-trips on "info threads."

To use this feature, (k)gdb needs to be built with the --with-expat option.
I would encourage enabling this option by default in our GDB port, if it is
not already.

Finally, there is another optional attribute you can specify per-thread
called a "handle." Handles are arbitrarily long sequences of bytes,
represented in the XML as hexadecimal. It is unclear to me how or if GDB
actually uses handles for anything. So I have left them out.