[gn] Add generic subcommands and provide list command

infra/mb/mb_config.pyl

 # Copyright 2016 The V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   # This is a map of buildbot master names -> buildbot builder names ->
   # config names (where each config name is a key in the 'configs' dict,
   # below). MB uses this dict to look up which config to use for a given bot.
   # Bots are ordered by appearance on waterfall.
   'masters': {
     'developer_default': {
       'arm.debug': 'default_debug_arm',
       'arm.optdebug': 'default_optdebug_arm',
       'arm.release': 'default_release_arm',
       'arm64.debug': 'default_debug_arm64',
       'arm64.optdebug': 'default_optdebug_arm64',
       'arm64.release': 'default_release_arm64',
+      'ia32.debug': 'default_debug_x86',
+      'ia32.optdebug': 'default_optdebug_x86',
+      'ia32.release': 'default_release_x86',
       'x64.debug': 'default_debug_x64',
       'x64.optdebug': 'default_optdebug_x64',
       'x64.release': 'default_release_x64',
-      'x86.debug': 'default_debug_x86',
-      'x86.optdebug': 'default_optdebug_x86',
-      'x86.release': 'default_release_x86',
     },
 
     'client.dart.fyi': {
       'v8-linux-release': 'gyp_release_x86_disassembler',
       'v8-win-release': 'gyp_release_x86_disassembler',
       'v8-mac-release': 'gyp_release_x86_disassembler',
     },
     'client.dynamorio': {
       'linux-v8-dr': 'gyp_release_x64',
     },
@@ skipping 647 matching lines @@
       'gn_args': 'target_cpu="x64"',
       'gyp_defines': 'target_arch=x64',
     },
 
     'x86': {
       'gn_args': 'target_cpu="x86"',
       'gyp_defines': 'target_arch=ia32',
     },
   },
 }