s390: new contributions GPLv2 or later

s390: new contributions GPLv2 or later

IBMs s390 contributions were meant to to be gplv2 or later (since
we were contributing to qemu). Several of the s390 specific files
link to gpl code anyway, so lets clarify the licence statement for
new contributions for those files that we have touched multiple
times or will likely touch again.

This patch does not touch files that mostly deal with tcg.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>