Enable EH on PPC Darwin. This basically works; there

are a couple of issues that show up with the optimizer,
but I don't think they're really EH problems.
(llvm-gcc testsuite users note:  By default the testsuite
uses the unwinding code that's built as part of your local
llvm-gcc, which does not work.  You need to trick it into
using the installed system unwinding code to get useful
results.)

llvm-svn: 45221