libcCross: set to `null` rather than using `assert` in nonsense scenarios

As discussed in

  https://discourse.nixos.org/t/help-me-understand-two-load-bearing-assert-statements/51420

The assertion here tickles (via wine, which is always the biggest headache for
our cross-compilation machinery) a `tryEval` that silently drops any outputs
that `throw` while splicing.

In this case, the `gcc.libgcc` output is `throw`ing due to checking if
`libcCross==null`.  The same effect is had by simply setting `libcCross` to
`null` in that situation, and this way we don't have to lean on `tryEval`.

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