Fix running Cranelift on benchmarks with LTO

[Kobzol]

Cranelift cannot currently handle lto = "thin" (@bjorn3 right?), so it fails on benchmarks which use it. We should either disable LTO or (ideally) skip running Cranelift with such benchmarks.

[bjorn3]

Indeed, LTO is not supported by cg_clif. Cranelift doesn't do any interprocedural optimizations right now. I think we should skip all non-check/debug benchmarks. Opt wouldn't be comparable anyway and doc shouldn't be affected by cg_clif in any way that check isn't too. And check is pretty much only affected by having to dlopen cg_clif and (in the past) using a different metadata loader.