[Bug sanitizer/78267] [7 Regression] libsanitizer breaks bootstrap on x86_64-apple-darwin16 at r241977

[howarth.at.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78267

--- Comment #30 from Jack Howarth <howarth.at.gcc at gmail dot com> ---
Created attachment 40035
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40035&action=edit
proposed fix for fixing darwin15 and earlier bootstraps