s390: always set -march compiler option

Currently we only set the -march compiler option if the kbuild
system figured out that the compiler actually supports the selected
architecture (cc-option test).

In result this means that no -march compiler option is set when an
unsupported cpu architecture of the current compiler is selected.
The kernel compile will afterwards succeed but with the default
architecture instead of the (unsupported) selected one.

Change this behaviour, so compiles will fail if the compiler does
not support the selected cpu architecture.

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