s390/kernel: build a relocatable kernel

This patch adds support for building a relocatable kernel with -fPIE.
The kernel will be relocated to 0 early in the boot process.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>