Force building with GCC 5 (#28098)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28098
Make sure that we're building with GCC 5 everywhere
ghstack-source-id: 92013998
Test Plan: waitforsandcastle
Differential Revision: D17953640
fbshipit-source-id: 26d978c60fc973c787383297d730b45d40fa300b
