Document using Hipcheck on older Linux distros

[alilleybrinker]

With our recent move to use Ubuntu 22.04 on our GitHub runners, a response to GitHub phasing out support for the Ubuntu 20.04 runners, we're now more likely to encounter cases of people trying to run Hipcheck on Ubuntu or other Linux distros with older versions of glibc.

The glibc version on the Ubuntu 22.04 runner is 2.35-0ubuntu3.9. (Source) So anything running on that version or newer is fine, since glibc is backwards-compatible; but anyone running on an older version will encounter strange issues. So far in practice we've seen weird hanging behavior.

We're not in a position right now to stand up our own self-hosted runners which would continue running on Ubuntu 20.04, so instead we'll need to figure out how to best document how to get Hipcheck working on older versions.

Some specific things I think we could do:

• [ ] Update the "Install" documentation to make clear that our Ubuntu builds will only work on Ubuntu systems with at least version 2.35 of glibc.
• [ ] Update the "Install" documentation to link to self-build instructions if the pre-built binary doesn't work for you.
• [x] Update self-build instructions with guidance for building on Ubuntu 20.04 and similar hosts; in particular we'll need to document how to turn off mold. We recently adopted mold as our linker on x86_64-unknown-linux-gnu, which has made our builds faster; but mold isn't available on Ubuntu 20.04, either in the official apt packages or as an independent PPA. Thankfully it is easy to turn off.
• [ ] Explore adding musl-based pre-built binaries with dist, so we can support Linux platforms without glibc, and also support running Hipcheck without relying on a system libc even on systems that do have glibc.