kernel: Allow k_thread_abort(_current) from ISRs

Traditionally k_thread_abort() of the current thread has done a
synchronous _Swap() to the new context.  Doing this from an ISR has
never worked portably (some architectures can do it, some can't) for
this reason.

But on Xtensa/asm2, exception handlers now run in interrupt context
and it's a very reasonable requirement for them to abort the excepting
thread.

So simply don't swap, but do the rest of the bookeeping, returning to
the calling context.  As a side effect it's now possible to terminate
threads from interrupts, even if they have been interrupted.

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