More improvements to GDBStub (#653)

* More improvements to GDBStub
- Debugging of threads should work correctly with source and assembly level stepping and modifying registers and memory, meaning threads and callstacks are fully clickable in VS.
- List of modules is available to the client, with assumption that .nro and .nso are backed up by an .elf with symbols, while deconstructed ROMs keep N names.
- Initial support for floating point registers.

* Tidy up as requested in PR feedback

* Tidy up as requested in PR feedback