The Amber branch will most likely not be seeing any support for new LLVM
versions. Go ahead and disable it like Gentoo does.

Frederik also wrote the following:

< freswa|M> foutrelis: Found this bug thread about disabling llvm in
                       mesa builds: https://bugs.freebsd.org/238906
< freswa|M> From a quick read llvm seems to be used in radeon gallium
            and vulkan drivers as well as in software acceleration as
            llvmpipe. Afaik those drivers are provided by mesa anyways.
            Software acceleration is a thing for VMs but people have no
            requirement for mesa-amber in those environments.