fix missing symbols when cross-compiling

[izgzhen]

fixes #581

[izgzhen]

I am actually reverting a change I've done in https://github.com/snowleopard/hadrian/pull/401/files. I tried to understand what is my motivation but found it hard to justify this by comparing to the latest HEAD. I think we can merge this as long as CI passed and no one jumps out and objects it.

[angerman]

These two lines (cross, compilerOrGhc) can be removed now as well, right?

[angerman]

I think we can drop some more dead code, no? Otherwise LGTM.

[snowleopard]

I think we should just make one of the CI instances (e.g. the selftest one) fail if there are any warnings. In this case there are warnings about unused code, which is difficult to spot while reviewing a PR, but easy to check by the compiler. Perhaps, we could also hook up @ndmitchell's Weeder :)

[izgzhen]

@angerman I've updated the code, thanks! I guess there will inevitably be some dead code, but let's do it when we have to.

@snowleopard Dead code check is a good idea! #584

[snowleopard]

@izgzhen Thanks, merged!