Default -fabi-version=0 for 4.9

[Jason Merrill <jason at redhat dot com>]

Our policy on mangling bugs has been that we don't change the mangling 
unless users explicitly specify -fabi-version.  Over time, this means 
that quite a few bugs have been found but continue to accumulate.  Most 
of these are C++11-specific, which means that as users use C++11 more 
frequently, the bugs are more likely to affect code in the wild.

For templates, it also seems to me that mangling changes have very 
little negative impact; in most cases the worst that will happen is that 
a program ends up with two compatible versions of the same function. 
And none of the changes affect libstdc++.so.  For non-template code, any 
failures will be at link time, making them straightforward to deal with.

The only non-mangling ABI change since version 2 is to argument 
promotion of C++11 scoped enums, and since we've been telling people 
that they need to rebuild all their C++11 code with every major release 
anyway, I don't think that's an obstacle.

So, for GCC 4.9 I would like to propose that we switch the default ABI 
version to 0 so that by default we use the most correct mangling.  If 
people really need backward compatible mangling for some reason they can 
specify the flag appropriately.

Jason