v2 builder + dummy-ghc: enable shared libs for wasm 9.12+ so TH works
Wasm 9.12+ GHC's RTS linker only supports shared libraries, and TH-eval's dyld needs a `.so` for every transitively reachable lib. Three connected fixes:
* `lib/dummy-ghc.nix`: real wasm 9.12+ GHC reports `target RTS linker only supports shared libraries: YES`; dummy was lying with `NO`. Per-version-gated so 9.6 / 9.8 / 9.10 wasm keep the legacy value.
* `test/dummy-ghc-info/default.nix`: un-ignore `target RTS linker only supports shared libraries` so the diff test catches this kind of divergence going forward. The prior comment claimed cabal didn't consult the field — that's true for the `pkgHashConfigInputs` hashing step, but the field still matters for shared-vs-static elaboration and is load-bearing for any other check that compares dummy vs real.
* `builder/comp-v2-builder.nix`: cabal's `--enable-shared` decision is driven by the `Support shared libraries` field (which real wasm GHC doesn't set), so plan-nix still records `--disable-shared` for wasm packages even with the dummy fix. v1 papered over this with the `enableShared || isWasm` clause in `comp-builder.nix:44`; v2 reads `configure-args` straight from plan-nix, so override the resulting pragma here to `shared: True` for wasm ≥ 9.12. Without this, TH-evaluating modules (e.g. `th-orphans`) fail at compile time with `dyld.findSystemLibrary(libHS…-…so): not found` plus `wasm-ld: error: unable to find library -lHS…-ghc<v>`.
* `builder/build-cabal-slice.nix`: add `gawk` to nativeBuildInputs. Darwin's stdenv `setup.sh` calls `awk` during fixup but darwin's default sandbox PATH doesn't carry it; build-cabal-slice's own diagnostics also use `awk`.
Verified on `aarch64-darwin.unstable.ghc9124.wasi32.tests.js-template-haskell.build` and `aarch64-darwin.unstable.ghc9124.{wasi32,native}.tests.dummy-ghc-info`.
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