s390/percpu: use generic percpu ops for CONFIG_32BIT

Remove the special cases for the this_cpu_* functions for 32 bit
in order to make it easier to add additional code for 64 bit.
32 bit will use the generic implementation.

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