dummy-ghc: align --info with real GHC across more compiler/cross variants
Extend `lib/dummy-ghc.nix` and the `tests.dummy-ghc-info` filter
list so the dummy's `--info` matches the real cross/native GHC
`--info` for the variants tested in hydra:
* aarch64-multiplatform (linux-gnu cross, Stage 1, no `_dyn`
RTS ways) — add a cross-linux-gnu branch alongside
android/static, keyed off cpu mismatch so libc-only crosses
(musl64) fall through to the native branch.
* `cross compiling` field now compares cpu + kernel rather
than the full platform triple, matching what GHC itself
reports (libc differences alone are not cross).
* musl32 (i686-linux-musl) and ucrt64 (x86_64-w64-mingw32):
`platformString` normalises `i686 -> i386` and the
Windows vendor/kernel pair to `unknown/mingw32`, matching
real GHC's `Target platform` / `target platform string`.
* ghc967 native and musl64: branch the RTS-ways string on
GHC version (9.6 uses a smaller, differently-ordered set
than 9.8+) and on libc (the 9.6 musl bootstrap reports a
12-way set while stock nixpkgs 9.6.7 reports 10).
Test-side: add a small set of per-version cosmetic fields to
`ignoredFields` (`GCC extra via C opts`, `LLVM clang command`,
`ld command`/flags, `dllwrap`, `touch command`,
`target RTS linker only supports shared libraries`) that don't
drive cabal-install elaboration.
Verified `tests.dummy-ghc-info` passes on linux-0 (with
`--builders ''`) for: ghc9141 native + aarch64-multiplatform,
ghc9124 musl32 + ucrt64, ghc967 native + musl64.