Make cpu_set_user_tls() work when called on the running thread.

On all the other architectures, this function can also be called on the
currently running thread. In this case, we shouldn't fix up the address
in the PCB, but also patch up the register itself. Otherwise it will not
become active and will simply become overwritten by the next switch.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D7437