I don't understand how it was ever possible to change the registers on a
thread that's not the current active one when a halt happened. Really
instead of the RTOS tracking what the currently selected thread is, it
would make more sense to have gdb_server do that and simply pass it
along in every call to the RTOS layer.
Now MulticoreRegTest passes.
Change-Id: I399b9b2b05a147aa6b41463714ed3a39534b1fc8