Default to building for the same architecture as the build host.

build/common.gypi

 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   'variables': {
     # .gyp files should set chromium_code to 1 if they build Chromium-specific
@@ skipping 17 matching lines @@
       # Override buildtype to select the desired build flavor.
       # Dev - everyday build for development/testing
       # Official - release build (generally implies additional processing)
       # TODO(mmoss) Once 'buildtype' is fully supported (e.g. Windows gyp
       # conversion is done), some of the things which are now controlled by
       # 'branding', such as symbol generation, will need to be refactored based
       # on 'buildtype' (i.e. we don't care about saving symbols for non-Official
       # builds).
       'buildtype%': 'Dev',
 
-      # The architecture that we're building on.
-      'target_arch%': 'ia32',
+      # Compute the architecture that we're building for. Default to the
+      # architecture that we're building on.
+      'conditions': [
+        [ 'OS=="linux"', {
+          # This handles the Linux platforms we generally deal with. Anything
+          # else gets passed through, which probably won't work very well; such
+          # hosts should pass an explicit target_arch to gyp.
+          'target_arch%':
+            '<!(uname -m | sed -e "s/i.86/ia32/;s/x86_64/x64/;s/arm.*/arm/")'
+        }, {  # OS!="linux"
+          'target_arch%': 'ia32',
+        }],
+      ],
 
       # We do want to build Chromium with Breakpad support in certain
       # situations. I.e. for Chrome bot.
       'linux_chromium_breakpad%': 0,
       # And if we want to dump symbols.
       'linux_chromium_dump_symbols%': 0,
       # Also see linux_strip_binary below.
 
       # By default, Linux does not use views. To turn on views in Linux,
       # set the variable GYP_DEFINES to "toolkit_views=1", or modify
@@ skipping 868 matching lines @@
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }
 
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