[Bug bootstrap/81037] Xcode 9 requires back ports on gcc-5-branch for bootstrapping under Xcode 9

[howarth.at.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81037

--- Comment #8 from Jack Howarth <howarth.at.gcc at gmail dot com> ---
Verified that proposed changes in PR81037_gcc5.patch with Xcode 8.2.1 on
darwin15.6.0 and Xcode 9 beta 5 on 10.13.