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

[jhb]

Summary

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.

Test Plan

• booted mips64 under QEMU which does not implement UL and requires the hot patching to work

[alfredo.mazzinghi_cl.cam.ac.uk]

Looks good to me.