SVC: Implemented GetThreadId.

For now threads are using their Handle value as their Id, it should not really cause any problems because Handle values are unique in Citra, but it should be changed. I left a ToDo there because this is not correct behavior as per hardware.
Please use Doxygen comments when documenting functions in headers. There are two formats, 1) the short-hand version (AKA the lazy version) that we use for other functions in this module, where it's just a one line explanation with the comment beginning with `///`. The other is the descriptive docstring format, designated with `/** ... */`, with arguments, return values, etc. clearly documented (here is a good example -> https://github.com/citra-emu/citra/blob/master/src/core/file_sys/archive_romfs.h#L31). I reccommend using the non-lazy format wherever possible, but at the very least use `///`.