Add illumos target

Author: Stjepan Glavina

.github/workflows/cross.yaml

@@ -45,3 +45,7 @@ jobs:
     - name: iOS
       if: startsWith(matrix.os, 'macos')
       run: cross build --target aarch64-apple-ios
+
+    - name: illumos
+      if: startsWith(matrix.os, 'ubuntu')
+      run: cross build --target x86_64-unknown-illumos