configure: Only build the s390-ccw bios if the compiler supports -march=z900

configure: Only build the s390-ccw bios if the compiler supports -march=z900

We want to build our s390-ccw bios with -march=z900 so that it also
works with the oldest s390x CPU that we support with TCG. However,
Clang on s390x does not support -march=z900 anymore, so we can not
use this compiler to build the s390-ccw bios. Thus add a proper test
to the configure script to see whether the compiler is usable.

Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1547470346[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>