Switching the default for -fabi-version

[Benjamin Kosnik]

Mark, I think this is a great idea, and am in favor of this switch. 

I think sufficiently clever system integrators will be able to rig a
gcc-3.4 so that it is multi-abi: ie a gcc-3.4-fabi=2 with
libstdc++.so.6, and gcc-3.4-fabi=1 with libstdc++.so.5 (from the last
stable branch.) I believe this will be able to keep compatibility
with..... gcc-3.3.x.... and still allow newly-compiled code to take
advantage of the newer runtime ABI and bug-fixed compiler ABI.

best,
benjamin