Proposed change: weak symbol support for Darwin

[Mark Mitchell]

> The only reason I was reluctant to make the change globally was that
> on most platforms (e.g. Linux/x86) the current scheme ain't broke.

For right or wrong, the ABI requires that all vtables with external
linkage have "vague linkage".  Therefore, I don't think we should change
the linkage of vtables on any platform, except those that don't have
proper support for weak/COMDAT/etc.

-- 
Mark Mitchell <mitchell@doubledemon.codesourcery.com>
CodeSourcery, LLC