s390/test_unwind: fix CALL_ON_STACK tests

The CALL_ON_STACK tests use the no_dat stack to switch to a different
stack for unwinding tests. If an interrupt or machine check happens
while using that stack, and previously being on the async stack, the
interrupt / machine check entry code (SWITCH_ASYNC) will assume that
the previous context did not use the async stack and happily use the
async stack again.

This will lead to stack corruption of the previous context.

To solve this disable both interrupts and machine checks before
switching to the no_dat stack.

Fixes: 7868249f ("s390/test_unwind: add CALL_ON_STACK tests")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>