Note that the tp register (r13) is reserved as the TLS pointer in
the same way that that gp register (r1) is reserved as the global
pointer. This implementation uses the tp register to point to the
thread structure used by the threads implementation. This is not
in violation with the runtime specification provided the TLS is
a fixed distance from the thread structure. This is only an issue
when code used the __thread keyword to create TLS. This is not
supported at the moment.
