GDB stub enhancements

[mfortune]

The GDB stub in MIPS qemu does not have XML support currently and is therefore limited to the classic fixed remote protocol register packet layouts. For FP64 and MSA support new packet layouts are defined using XML. I have been hacking in support for this while testing GDB support for the same and can offer my initial work as a starter.
The new register layouts are not documented yet but I'll add a reference to the spec once it is written up.