[Bug bootstrap/82300] Bootstrapping gcc-8 does not work on darwin

[dominiq at lps dot ens.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82300

--- Comment #3 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
If this PR appears at or before r 252896, The problem I see is different.