wrapCC should use buildPackages

[matthewbauer]

Issue description

Currently we don't have good support for cross compiling when we use wrapCC. We should ensure that buildPackages is always used correctly here.

/cc @Ericson2314

[Ericson2314]

Right now, compilers and binutils are wrapped with libraries from the next stage (targetPackages), and stdenvs take all their tools from the previous stage (buildPackages). For the moment I think it's easiest to continue with that. But a super early cross attempt of mine #21268 put the wrapper in the middle stage instead.

Once libc is not special I was thinking it would be a good time to revisit such things.

[stale]

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