Cannot cross-compile from x86_64 to i686 with LLVM 8

[RalfJung]

As @eddyb found out, #311 introduced a regression on old versions of LLVM: cross-compilation to i686-unknown-linux-gnu on an x86_64 host is broken. In particular this currently prevents us from ensuring 32bit tests work on PR CI.

[RalfJung]

One option might be to simply revert that PR, since (from what I can see in rust-lang/rust#58599) the unordered intrinsics are not actually used. But then I wonder why they lead to problems.

[Mark-Simulacrum]

It seems like at least gating them on LLVM >= 9 or whatever would be reasonable...

[RalfJung]

Does compiler-builtins have infrastructure for LLVM version gating?

[Amanieu]

Rust no longer supports LLVM 8.