24k errata fixes

[Richard Sandiford]

Catherine Moore <clm@codesourcery.com> writes:
> Thanks for the updates to the errata patch.  I'm curious!  Did you 
> encounter a problem that caused you to revisit this issue?

Yeah, I've been looking at some ways of cleaning up append_insn,
but hit some unexpected fallout in the 24k tests.  Which just
goes to show that they're doing their job :-)

> Your changes look reasonable to me.

Great, thanks.

Richard