MacOS APFS libstdc++ build workaround

[chrisj at rtems.org]

Hi,

I will push this patch. It is needed on some MacOS system where the APFS file
system and building GCC's libstdc++ fail.

This patch was in the gcc-7.3.0 builds so it would be good if this patch could
move with gcc as it changes. I doubt this part of gcc build system will change
anytime soon.

Thanks
Chris