Simplify hot-patching cpu_switch() for lack of UserLocal register.

Rather than walking all of cpu_switch looking for the sequence of
instructions to patch, add a global label at the location that needs
the patch applied.

Reviewed by: brooks, Alfredo Mazzinghi <alfredo.mazzinghi_cl.cam.ac.uk>
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24931