Add mac as target_os and mac_cpu (#25810)

https://github.com/flutter/flutter/issues/69221

This allows creating build folders for macOS cross compilation

tools/gn

@@ -39,6 +39,9 @@ def get_out_dir(args):
     if args.ios_cpu != 'arm64':
         target_dir.append(args.ios_cpu)
 
+    if args.mac_cpu != 'x64':
+      target_dir.append(args.mac_cpu)
+
     if args.simulator_cpu != 'x64':
         target_dir.append(args.simulator_cpu)
 
@@ -183,6 +186,8 @@ def to_gn_args(args):
             gn_args['target_cpu'] = args.simulator_cpu
         else:
             gn_args['target_cpu'] = args.ios_cpu
+    elif args.target_os == 'mac':
+      gn_args['target_cpu'] = args.mac_cpu
     elif args.target_os == 'linux':
       gn_args['target_cpu'] = args.linux_cpu
     elif args.target_os == 'fuchsia':
@@ -350,11 +355,13 @@ def parse_args(args):
   parser.add_argument('--full-dart-debug', default=False, action='store_true', help='Implies --dart-debug ' +
       'and also disables optimizations in the Dart VM making it easier to step through VM code in the debugger.')
 
-  parser.add_argument('--target-os', type=str, choices=['android', 'ios', 'linux', 'fuchsia', 'win', 'winuwp'])
+  parser.add_argument('--target-os', type=str, choices=['android', 'ios', 'mac', 'linux', 'fuchsia', 'win', 'winuwp'])
   parser.add_argument('--android', dest='target_os', action='store_const', const='android')
   parser.add_argument('--android-cpu', type=str, choices=['arm', 'x64', 'x86', 'arm64'], default='arm')
   parser.add_argument('--ios', dest='target_os', action='store_const', const='ios')
   parser.add_argument('--ios-cpu', type=str, choices=['arm', 'arm64'], default='arm64')
+  parser.add_argument('--mac', dest='target_os', action='store_const', const='mac')
+  parser.add_argument('--mac-cpu', type=str, choices=['x64', 'arm64'], default='x64')
   parser.add_argument('--simulator', action='store_true', default=False)
   parser.add_argument('--linux', dest='target_os', action='store_const', const='linux')
   parser.add_argument('--fuchsia', dest='target_os', action='store_const', const='fuchsia')