[S390] smp: fix lowcore allocation

The intermediate lowcore for CONFIG_SMP is allocated using a call to
__alloc_bootmem() with a goal of 0. That however doesn't guarantee that
the allocated piece of memory is below 2GB.
Instead we should call __alloc_bootmem_low().

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>