kernel/spinlock: Fix SPIN_VALIDATE in ISRs

Spinlocks taken in ISRs were storing the _current thread pointer of
the interrupted thread as the owner, which was never strictly correct
but was benign as the thread would never run until the lock was
released.

But now k_thread_abort(_current) in an ISR has been fixed to eliminate
all references to the (now aborted) thread struct, and _current points
to a dummy thread.  Handle that edge case in the validation framework.

Signed-off-by: Andy Ross <andyross@google.com>