[SystemZ] Allocate a second register scavenging slot

This is another prerequisite for frame-to-frame MVC copies.
I'll commit the patch that makes use of the slot separately.

The downside of trying to test many corner cases with each of the
available addressing modes is that a fair few tests need to account
for the new frame layout.  I do still think it's useful to have all
these tests though, since it's something that wouldn't get much coverage
otherwise.