lib/systems/parse.nix: mkSkeletonFromList: improve readability

The main purpose of this PR is to make the basis for
`mkSkeletonFromList`'s decision between `cpu-kernel-libcabi` vs
`cpu-vendor-os` clear, without changing its behavior.  The existing
code obscures this decision behind a sequence of prioritized matches
(i.e. `if-then`) which jump around between different coordinates.

Two side benefits of this PR:

1. It makes the root cause of #165836 obvious: we are missing a case
   for `cpu-vendor-libcabi`.  This is why nixpkgs stumbles over
   `*-none-*`.

2. It illuminates some very weird corner cases in the existing
   logic, like `*-${vendor}-ghcjs` overriding the `vendor` field,
   and `mingw32` being transformed into `windows` in some cases.

Co-authored-by: John Ericson <git@JohnEricson.me>

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