[libc] Change minimum cmake requirement.

Summary: As per this thread: http://lists.llvm.org/pipermail/llvm-dev/2020-April/140744.html, changed the minimum requirement to reflect the decision made to upgrade to cmake 3.13.4.

Reviewers: sivachandra

Reviewed By: sivachandra

Subscribers: mgorny, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D77781