Driver/DarwinClang: The new toolchain definition is going to drop the -L inside

Driver/DarwinClang: The new toolchain definition is going to drop the -L inside
the GCC dir. Unfortunately, this breaks -lstdc++ on SnowLeopard, etc. because
the libstdc++ dylib was hiding there. Workaround this by providing the path to
the right -lstdc++.6 (the only version used in recent memory) if we can't see an
obvious -lstdc++, but can find = -lstdc++.6.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114146 91177308-0d34-0410-b5e6-96231b3b80d8