Switching the default for -fabi-version

[law@redhat.com]

In message <1066788159.26681.21.camel@doubledemon.codesourcery.com>, Mark Mitch
ell writes:
 >The short answer is "in some classes, some fields will be at different
 >offsets".  But that sounds much more severe than it really is; the
 >longer answer is "in a few corner-case classes -- such as those with
 >lots of empty base classes, or those that have base classes that end
 >with bitfields -- the offsets of some virtual bases and/or fields might
 >change."
You might want to talk the Mozilla guys -- I've heard them say that they
heavily use empty base classes.  They might be able to do some testing
to see if their code is affected.  Does -Wabi catch the empty base class
changes?

 >There are also a few places (involving complex templates) where the
 >mangled name of a function changed.
Not sure who'd have the best real-world tests for this.  

Jeff