musl host support: bootstrap and run on Alpine (x86_64-alpine-linux-musl)

The Linux host code paths were gated on the x86_64_pc_linux_gnu target
define alone, and LoadProject hardcoded --target=x86_64-pc-linux-gnu when
compiling project.cpp and the module PCMs. Both break on musl hosts: the
glibc release launcher cannot run there at all (no glibc loader; gcompat
lacks libgcc_s and the __isoc23_* symbols), and a musl-built launcher
would still compile project.cpp for the wrong libc.

- The host gates test clang's __linux__ instead of one triple's define:
  they mean "Linux host", whatever the libc (or, later, the CPU).
- LoadProject derives the host triple from HostTarget() (clang's
  -print-target-triple), which is also more correct on glibc distros
  whose triple isn't x86_64-pc-linux-gnu.
- project.cpp / the cmake libc++ flags treat any *-linux-gnu or
  *-linux-musl target as Linux.
- build.sh targets the host toolchain's triple (CRAFTER_BUILD_TARGET
  overrides) and derives the per-target define from it. On Arch this is
  byte-for-byte the previous behaviour.
- CI: a release-musl job bootstraps on alpine:edge, runs the tests, and
  attaches crafter-build-linux-x86_64-musl-v2.tar.gz to the rolling
  latest release (via the API, so the glibc assets survive). Dynamic
  against musl libc++: the launcher dlopens project.so, so it cannot be
  fully static.

First consumer: imsd's package CI, which cross-compiles for aarch64 from
an Alpine container.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.forgejo/workflows/ci.yaml

@@ -186,3 +186,76 @@ jobs:
           override: true
           release-dir: dist
           token: ${{ secrets.GITHUB_TOKEN }}
+
+  # musl build for Alpine / postmarketOS hosts: the glibc launcher above cannot
+  # run there (musl has no glibc loader and gcompat lacks libgcc_s + the
+  # __isoc23_* symbols). Bootstrap natively on alpine:edge and ship the
+  # bootstrap output — the same thing the Arch PKGBUILD installs. Dynamic
+  # against musl libc++ (crafter-build dlopens project.so, so it cannot be
+  # fully static); consumers need `apk add clang lld libc++-dev llvm-runtimes`.
+  release-musl:
+    needs: build-test-release
+    if: (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == 'refs/heads/master'
+    runs-on: arch-latest
+    container:
+      image: alpine:edge
+    env:
+      CRAFTER_BUILD_MARCH: x86-64-v2
+      CRAFTER_BUILD_MTUNE: generic
+    steps:
+      - name: Install build dependencies
+        # nodejs: actions/checkout is a Node action. build-base: gcc's
+        # libgcc_s + crt objects, which clang's musl driver links against, plus
+        # make/binutils for the glslang cmake build (python3: its generators).
+        # clang-dev + llvm-dev: the libclang header Crafter.Build-Lint includes
+        # lives under llvm-config --includedir here, not /usr/include. llvm:
+        # llvm-ar for the LTO static archives the self-host/test builds write.
+        run: |
+          apk add -q nodejs git curl jq tar clang clang-dev lld libc++-dev \
+            llvm-libunwind-dev llvm-runtimes llvm llvm-dev cmake build-base python3
+          git config --global --add safe.directory '*'
+
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Bootstrap (build.sh)
+        run: ./build.sh
+
+      - name: Run tests
+        run: CRAFTER_BUILD_HOME=$PWD/share/crafter-build ./bin/crafter-build test
+
+      - name: Package artifact
+        run: |
+          set -eux
+          file bin/crafter-build || true
+          mkdir -p dist
+          stage=$(mktemp -d)
+          mkdir -p "$stage/bin" "$stage/lib"
+          cp bin/crafter-build "$stage/bin/"
+          cp lib/libcrafter-build.a "$stage/lib/"
+          cp -r share "$stage/"
+          tar czf dist/crafter-build-linux-x86_64-musl-v2.tar.gz -C "$stage" .
+          ls -la dist/
+
+      - name: Upload musl artifact
+        uses: actions/upload-artifact@v3
+        with: { name: crafter-build-linux-x86_64-musl-v2, path: dist/crafter-build-linux-x86_64-musl-v2.tar.gz, if-no-files-found: error }
+
+      # Add the asset to the rolling 'latest' release the first job just
+      # (re)published, via the API rather than forgejo-release: that action's
+      # override mode would drop the other job's assets.
+      - name: Attach to rolling 'latest' release
+        env:
+          TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          API: ${{ github.server_url }}/api/v1/repos/${{ github.repository }}
+        run: |
+          set -eu
+          asset=crafter-build-linux-x86_64-musl-v2.tar.gz
+          rel=$(curl -fsS -H "Authorization: token $TOKEN" "$API/releases/tags/latest")
+          id=$(printf '%s' "$rel" | jq -r .id)
+          for old in $(printf '%s' "$rel" | jq -r --arg n "$asset" '.assets[] | select(.name==$n) | .id'); do
+            curl -fsS -X DELETE -H "Authorization: token $TOKEN" "$API/releases/$id/assets/$old"
+          done
+          curl -fsS -H "Authorization: token $TOKEN" -F "attachment=@dist/$asset" \
+            "$API/releases/$id/assets?name=$asset" > /dev/null
+          echo "attached $asset to release $id"