arch/xtensa: Fix outgoing stack flush for dummy threads

On CPU startup, When we reach the cache flush code in arch_switch(),
the outgoing thread is a dummy.  The behavior of the existing code was
to leave the existing value in the SR unchanged (probably NULL at
startup).  Then the context switch would walk from that address up to
the top of the outgoing stack, flushing everything in between.  That's
wrong, because the outgoing stack is a real pointer (generally the
interrupt stack of the current CPU), and we're flushing everything in
memory underneath it.

This also reverts commit 29abc8ad ("xtensa: fix booting secondary
cores on the dummy thread"), which appears to have been an early
attempt to address this issue.  It worked (modulo all the extra and
potentially incorrect flushing) on cavs v1.5/1.8 because of the way
the entry code worked there.  But on 2.5 we now hit the first context
switch in a case where those extra lines are in address space already
marked unwritable by the CPU, so the flush explodes.

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