s390/nmi: use the normal asynchronous stack for machine checks

If a machine checks is received while the CPU is in the kernel, only
the s390_do_machine_check function will be called. The call to
s390_handle_mcck is postponed until the CPU returns to user space.
Because of this it is safe to use the asynchronous stack for machine
checks even if the CPU is already handling an interrupt.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>