s390: add z13 code generation support

Allow to generate code that only runs on z13 machines.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --cc arch/s390/Kconfig
Simple merge

diff --cc arch/s390/Makefile
Simple merge

diff --cc arch/s390/kernel/head.S
Simple merge

diff --cc arch/s390/kernel/setup.c
Simple merge