ci: Add job for Apple Silicon

This marks the beginning of transitioning to the arm64 architecture as
the default. The x86_64 job remains present but is being phased out of
24/7 operations, i.e. the job is now on-demand only.

.gitlab-ci.yml

@@ -210,34 +210,46 @@ msys2-mingw64:
     paths:
       - "${CI_PROJECT_DIR}/_build/gtkdll.tar.gz"
 
-macos-x86_64:
+macos:
   rules:
     # Do not run in forks as the runner is not available there.
     # (except for dehesselle who maintains the runner)
-    - if: $CI_PROJECT_NAMESPACE == "GNOME"
-    - if: $CI_PROJECT_NAMESPACE == "dehesselle"
+    - if: $CI_PROJECT_NAMESPACE != "GNOME" && $CI_PROJECT_NAMESPACE != "dehesselle"
+      when: never
+    - if: $RUNNER == "macosintel"
+      variables:
+        SDKROOT: /opt/sdks/MacOSX10.13.4.sdk
+      when: manual
+      allow_failure: true
+    - if: $RUNNER == "macosarm"
+      variables:
+        SDKROOT: /opt/sdks/MacOSX11.3.sdk
   stage: build
+  parallel:
+    matrix:
+      - RUNNER: [ "macosintel", "macosarm" ]
   tags:
-    - macosintel
+    - ${RUNNER}
   needs: []
   variables:
     EXTRA_MESON_FLAGS: ""
     BACKEND_FLAGS: "-Dx11-backend=false -Dbroadway-backend=true"
     FEATURE_FLAGS: "-Dmedia-gstreamer=disabled -Dintrospection=enabled -Dgobject-introspection:werror=false"
     TMPDIR: /Users/Shared/work/tmp
-    SDKROOT: /opt/sdks/MacOSX10.13.4.sdk
     PIP_CACHE_DIR: /Users/Shared/build/cache
     PIPENV_CACHE_DIR: $PIP_CACHE_DIR
     PYTHONPYCACHEPREFIX: $PIP_CACHE_DIR
   before_script:
-    # Not using ccache on purpose as it accelerates the build so much that it
-    # can trigger race conditions in the gobject-introspection subproject.
     - bash .gitlab-ci/show-info-osx.sh
     - python3 -m venv .venv
+    # Building the introspection feature requires pkg-config and bison.
     - curl -L $CI_API_V4_URL/projects/30437/packages/generic/pkgconfig/v0.29.2+10/pkg-config-0.29.2+10_$(uname -m).tar.xz | tar -C .venv -xJ
     - curl -L $CI_API_V4_URL/projects/30438/packages/generic/bison/v3.8.2+3/bison-3.8.2+3_$(uname -m).tar.xz | tar -C .venv -xJ
     - source .venv/bin/activate
     - pip3 install meson==1.3.2 ninja==1.11.1.1
+    # We're not setting up ccache here on purpose as it accelerates the build
+    # so much that it triggers race conditions in the gobject-introspection
+    # subproject.
   script:
     - meson setup
             ${COMMON_MESON_FLAGS}