scripts: document how to build older versions on newer systems

scripts: document how to build older versions on newer systems

Based on https://lists.gnu.org/r/coreutils/2019-08/msg00011.html .

* scripts/build-older-versions/README.older-versions: Documentation
* scripts/build-older-versions/build-older-versions.sh: Helper script.
* scripts/build-older-versions/.gitignore: Ignore build directory.
* scripts/build-older-versions/coreutils-5.0-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-5.97-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-6.10-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-6.11-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-6.12-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-7.2-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-8.13-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-8.17-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-8.18-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-8.24-on-glibc-2.28.diff,
scripts/build-older-versions/coreutils-8.4-on-glibc-2.28.diff: Patches.