Support was previously added to gnu x86_64 and all musl targets but
others were not included because of a roundtrip issue [1].

This commit adds support for the ip_mreqn struct on all Linux GNU
targets which did not have the roundtrip issue.

[1]: https://github.com/rust-lang/libc/issues/1558



Signed-off-by: Bartel Sielski <bartel.sielski@gmail.com>

Reorganized some constants as well so they're grouped a bit better.