pkg/fff compiles fine with clang 16.0.0 and higher, but the CI currently
uses version 14.x.y which is incompatible. This disables LLVM in the CI
builds and prints a friendly error message when `TOOLCHAIN=llvm` is
used and clang is older than 16.0.0.
Makefiles don't do comments, so these were forwarded into the variable.
*Most* users would expand the arguments to a shell where it'd be
ignored, but not all of them.
Contributes-To: https://github.com/RIOT-OS/RIOT/pull/18489
(This is also where the one version that is added here was removed).
