Download script to download all libc versions

[goreil]

This pull request introduces a new script which downloads all relevant libc versions and helps address #180

[goreil]

The Idea of this helper script is that the relevant libc versions need to be downloaded before they can be linked to the versions.

It's also nice even when going the ./glibc_run.sh route, since the downloads are all frontloaded, instead of waiting times every time a new libc version is requested.

[Kyle-Kyle]

Thank you for the contribution, and sorry for the late response.
I'd prefer not to force all users to download all libc.
I'll make some changes so users can only download/link relevant libc versions.