[Bug bootstrap/89864] [9 regression] gcc fails to build/bootstrap with XCode 10.2

[schnetter at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89864

--- Comment #7 from Erik Schnetter <schnetter at gmail dot com> ---
I tried adding a fixinclude that #defines _Atomic to volatile if the system
header is included from C++, and this resolved the issue for me.

A possible implementation is described here
<https://github.com/spack/spack/issues/11042>. I plan to submit a proper patch
to GCC next week.