xtensa: move _SimulateUserKernelVectorException out of WindowVectors

In configurations without window registers support the section
.WindowVectors.text may never be linked.
_SimulateUserKernelVectorException is a common handler for high priority
interrupts, it does not belong in that section anyway. Move it out of
that section and mark it as __XTENSA_HANDLER so it gets bundled with
other vector helpers.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>