[cmake] Temporarily revert enforcement of minimum GCC version increase

Summary:
This is temporary, until bot that builds public facing LLVM
documentation is upgraded. It reverts only the cmake change in r284497,
but leaves the doc changes in place to preserve intent.

Reviewers: aaron.ballman

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26078

llvm-svn: 285406