GN: support 32-bit x86 builds

infra/bots/recipes/swarm_compile.expected/Build-Win-MSVC-x86-Release-GN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[CUSTOM_C:\\_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 116 matching lines @@
     "env": {
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "fetch-gn"
   },
   {
     "cmd": [
       "gn",
       "gen",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86-Release-GN\\Release",
-      "--args=is_debug=false"
+      "--args=is_debug=false target_cpu=\"x86\""
     ],
     "cwd": "[CUSTOM_C:\\_B_WORK]\\skia",
     "env": {
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gn gen"
   },
   {
     "cmd": [
       "ninja",
@@ skipping 60 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    pass@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]