I realized what rhelmot did in 61202561d9
(specify what packages just need `stdenvNoLibc`) is definitely the right
approach for this, and adjusted NetBSD and OpenBSD to likewise use it.
With that change, we don't need these confusing and ugly `*bsdCross`
package sets at all!
We can get rid of a lot more libc-related `*Cross`, and I will do so
soon, but this is the first step.
(adapted from commit 51f1ecaa59)
744d7e7477