s390/percpu: Provide arch_this_cpu_read() implementation

s390/percpu: Provide arch_this_cpu_read() implementation

Provide an s390 specific implementation of arch_this_cpu_read() instead
of the generic variant. The generic variant uses preempt_disable() /
preempt_enable() pair and READ_ONCE().

Get rid of the preempt_disable() / preempt_enable() pairs by providing an
own variant which makes use of the new percpu code section infrastructure.

With this the text size of the kernel image is reduced by ~1k
(defconfig). Also 87 generated preempt_schedule_notrace() function
calls within the kernel image (modules not counted) are removed.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>