Use LLVM Linker
Build LLVMs linker and use it in the smoketest (with the intention to use it also for any future code that would be part of repo).
In general any linker should work, but the GNU system linker version on the Dolent CI servers is too old, and doesn't recognize relocations used by LLVM 12. No such problems in LLVM 9, so the "issue" appeared in some LLVM version between those.
In any case, it seems reasonable to use the LLVM linker as default, as the compiler is then self-contained.