Apply fix from D81179 only from GCC < 8

Summary: Apply workaround done in D81179 only for GCC < 8. As @klausler mentioned in D81179 we want to avoid additional checks for other compilers that do not need them.

Reviewers: DavidTruby, klausler, jdoerfert, sscalpone

Reviewed By: klausler, sscalpone

Subscribers: llvm-commits, tskeith, isuruf, klausler

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D81208