ci: split the macOS jobs, and test only the host on the intel runner

The macos-15-intel job was the critical path of CI at 1h21m44s. It ran
gen-device, make test, make release, make tinygo-test, and
make smoketest-quick one after the other.

Split the work into four jobs, the same pattern as windows.yml.
build-macos publishes the release tarball, stdlib-test-macos and
smoke-test-macos download it, and test-macos restores the LLVM cache and
runs at the same time as build-macos. The LLVM steps move to a new
setup-llvm-macos composite action, with a save-cache input so that only
build-macos writes the cache.

Most of smoketest-quick is cross compilation, which gives the same result
on every host. Add a smoketest-host target with the host dependent part
plus a Cortex-M and a wasm canary, and use it on the intel runner. The
arm64 runner keeps the full set.

The intel runner still tests darwin/amd64 with make test, make
tinygo-test, and smoketest-host.

.github/actions/setup-llvm-macos/action.yml

@@ -0,0 +1,65 @@
+name: Set up LLVM on macOS
+description: Restore or build LLVM
+
+inputs:
+  os:
+    description: Runner image name, used in the cache key
+    required: true
+  save-cache:
+    description: Save LLVM caches after a cache miss
+    required: false
+    default: "true"
+
+runs:
+  using: composite
+  steps:
+    - name: Restore LLVM source cache
+      uses: actions/cache/restore@v5
+      id: cache-llvm-source
+      with:
+        key: llvm-source-22-${{ inputs.os }}-v1-${{ hashFiles('llvm-version.txt') }}
+        path: |
+          llvm-project/clang/lib/Headers
+          llvm-project/clang/include
+          llvm-project/compiler-rt
+          llvm-project/lld/include
+          llvm-project/llvm/include
+    - name: Download LLVM source
+      if: steps.cache-llvm-source.outputs.cache-hit != 'true'
+      shell: bash
+      run: make llvm-source
+    - name: Save LLVM source cache
+      uses: actions/cache/save@v5
+      if: inputs.save-cache == 'true' && steps.cache-llvm-source.outputs.cache-hit != 'true'
+      with:
+        key: ${{ steps.cache-llvm-source.outputs.cache-primary-key }}
+        path: |
+          llvm-project/clang/lib/Headers
+          llvm-project/clang/include
+          llvm-project/compiler-rt
+          llvm-project/lld/include
+          llvm-project/llvm/include
+    - name: Restore LLVM build cache
+      uses: actions/cache/restore@v5
+      id: cache-llvm-build
+      with:
+        key: llvm-build-22-${{ inputs.os }}-v1-${{ hashFiles('llvm-version.txt') }}
+        path: llvm-build
+    - name: Build LLVM
+      if: steps.cache-llvm-build.outputs.cache-hit != 'true'
+      shell: bash
+      run: |
+        # fetch LLVM source
+        rm -rf llvm-project
+        make llvm-source
+        # install dependencies
+        HOMEBREW_NO_AUTO_UPDATE=1 brew install ninja
+        # build!
+        make llvm-build
+        find llvm-build -name CMakeFiles -prune -exec rm -r '{}' \;
+    - name: Save LLVM build cache
+      uses: actions/cache/save@v5
+      if: inputs.save-cache == 'true' && steps.cache-llvm-build.outputs.cache-hit != 'true'
+      with:
+        key: ${{ steps.cache-llvm-build.outputs.cache-primary-key }}
+        path: llvm-build