need for flag for incompatible-changes

[Mike Stump]

If we have a flag, I think I prefer a more general solution where we
select a flag that specifies what abi we want, and then use that.
When we make incompatible changes, then we conditionalize the new code
on the new value for the flag, and use the old code (with the
disadvantages it brings), with the old value.

-fcompat-2.8.0 is default (for now), and generates code compatible
with 2.8.0, -fcompat-2.9.0 likewise.

I'm not sure if we should do this or not, though I have thought about
it on and off for a while now.

Another way, is to just have a #define NEW_EH and then switch it on at
some point in the future, and then remove it sometime later.