kernel/idle: Fix !SCHED_IPI_SUPPORTED

The requirement for k_yield() to handle "yielding" in the idle thread
was removed a while back, but it missed a spot where we'd try to yield
in the fallback loop on bringup platforms that lack an IPI.  This now
crashes, because yield now unconditionally tries to reschedule the
current thread, which doesn't work for idle threads that don't live in
the run queue.

Just make it a busy loop calling swap(), even simpler.

Fixes #50119

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