darwin may lose primary target status on FSF gcc

[Jack Howarth]

  There is hope! We have an analysis of the exact
breakage in eh under 10.6 for gcc trunk...

http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-September/025908.html
http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-September/025909.html

The breakage in gcc trunk is due to the additional epilog unwind
information which darwin's unwinder (which is always used under
10.6 since symbols in libgcc-10.5 always come from libSystem)
doesn't understand. The possible fixes are either to 1) not add
the additional epilog unwind information on darwin or 2) have
the gcc driver on darwin implicitly pass -no_compact_unwind to
the link line. Cheers.
           Jack