Remove buildable check based on dlfcn.h

[TerrorJack]

This PR is a predecessor to #205. We shall no longer deny building for platforms lacking dlfcn.h, but rely on fine-grained checks on individual interfaces instead.

[TerrorJack]

Hi, would you take another look at this PR? The aarch64 CI failure is irrelevant and seem to be random timeout, shouldn't be a problem if the 60min limit is bumped a bit.

[hs-viktor]

I am not sure why this makes sense as a stand-alone PR. The dlfcn.h header is still included in HsUnix.h, so the unix package won't build if it is missing without further changes. Why not make this change in a PR that actually makes unix not unconditionally depend on dlfcn.h.

[TerrorJack]

And that PR would be #205. @Bodigrim indeed the changes here are too small to be meaningful, would you agree on closing this one and getting back to #205 once the ghc warnings are fixed there?

[Bodigrim]

@TerrorJack no problem.