arch/xtensa: Invalidate bottom of outbound stacks

Both new thread creation and context switch had the same mistake in
cache management: the bottom of the stack (the "unused" region between
the lower memory bound and the live stack pointer) needs to be
invalidated before we switch, because otherwise any dirty lines we
might have left over can get flushed out on top of the same thread on
another CPU that is putting live data there.

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