xtensa/asm2: Fix stack pointer during preemption spills

When returning into a different thread than we interrupted, we
obviously need to spill all the existing register windows to make sure
all their values are in the old thread's stack.  But the code to do
this forgot to reset the current stack pointer to the value it had at
interrupt time (it was still pointing to the saved context below
that), so the caller of the interrupted function was spilling to the
wrong spot.

This wouldn't show up as an instant failure, it would only happen when
switching BACK to the improperly-spilled thread.  And even then it
would be a noop if the original interrupt handler was deep enough to
have spilled that function naturally.

In practice, this happened only in some instances on ESP-32 (which has
more windowed registers than qemu) when interrupting the idle thread
(which is very shallow) with a (very simple) timer interrupt.  Trivial
to see, hard to find.

See https://github.com/zephyrproject-rtos/zephyr/issues/6346

 for more
detail.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>