S390: Do not clobber r7 in clone [BZ #31402]

Starting with commit e57d8fc9
"S390: Always use svc 0"
clone clobbers the call-saved register r7 in error case:
function or stack is NULL.

This patch restores the saved registers also in the error case.
Furthermore the existing test misc/tst-clone is extended to check
all error cases and that clone does not clobber registers in this
error case.