Test against multiple stdlib implementations

[edsko]

As of #222 we only test against musl; the downside of this approach is that we are no longer testing hs-bindgen against different implementations of the C standard library, which is unfortunate. We may want to reconsider that aspect at some later stage.

[phadej]

It's relatively easy to add more libs. I didn't go for glibc, as then we probably need to separate tests into separate package so we don't distribute the glibc (even it its part of the tests).

[phadej]

And e.g. netbsd libc is probably a good idea to add, as it's a) free, b) iirc not very large, c) about as wonky as macos

[edsko]

Might also be worth looking at multiple musl libraries (that is, generated for different architectures).