automation/gitlab: introduce macOS build jobs

With the imminent decommission of CirrusCI we will lose the limited macOS
build testing done there.  Introduce two Gitlab macOS build jobs that
reproduce the ones present on CirrusCI.

According to the "Community programs" Gitlab documentation [0] we get
50,000 compute minutes per month.  The arm64 job takes ~1m 15s, and the
x86_64 one ~2m.  I think it's likely enough given the amount of minutes we
have available.  We might need to reconsider if we add more tasks that
consume compute minutes.

[0] https://docs.gitlab.com/subscriptions/community_programs/#gitlab-for-open-source

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

automation/gitlab-ci/build.yaml

@@ -815,3 +815,33 @@ debian-13-riscv64-gcc-randconfig:
     KBUILD_DEFCONFIG: tiny64_defconfig
     RANDCONFIG: y
     <<: *riscv-fixed-randconfig
+
+# macOS build jobs
+.macos-26:
+  <<: *build
+  tags:
+    - saas-macos-medium-m1
+  image: macos-26-xcode-26
+  variables:
+    HOMEBREW_NO_AUTO_UPDATE: 1
+    HOSTCC: clang
+    HYPERVISOR_ONLY: y
+    <<: *gcc
+  script:
+    - brew install ${CROSS_COMPILE}gcc ${CROSS_COMPILE}binutils
+    - alias nproc="sysctl -n hw.ncpu"
+    - ./automation/scripts/build 2>&1 | tee build.log
+
+macos-26-x86_64:
+  extends:
+    - .macos-26
+  variables:
+    XEN_TARGET_ARCH: x86_64
+    CROSS_COMPILE: x86_64-elf-
+
+macos-26-arm64:
+  extends:
+    - .macos-26
+  variables:
+    XEN_TARGET_ARCH: arm64
+    CROSS_COMPILE: aarch64-elf-