s390: Build for z10 instead of z196.

Upstream defconfig wants to optimize for z196; however, gcc 4.5 as
currently used doesn't support the z196 optimizations yet. Let's
switch to z10 instead which should be fine.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Merge pull request #8 from cohuck/s390-march

s390: Build for z10 instead of z196.