s390/smp: Avoid busy loop after halt and "begin" on z/VM

Currently the smp_stop_cpu() function for SMP kernels enters a busy
loop when "begin" is entered on the z/VM console after Linux is halted.
To avoid this behavior, use the non-SMP variant of smp_stop_cpu()
which stops the CPU again after "begin" is entered. As a side
effect we now have consistent behavior for SMP and non-SMP Linux.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>