s390/smp: cleanup target CPU callback starting

Macro mem_assign_absolute() is used to initialize a target
CPU lowcore callback parameters. But despite the macro name
it writes to the absolute lowcore only if the target CPU is
offline. In case the CPU is online the macro does implicitly
write to the normal memory.

That behaviour is correct, but extremely subtle. Sacrifice
few program bits in favour of clarity and distinguish between
online vs offline CPUs and normal vs absolute lowcore pointer.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>