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

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index b5af77ac16eaf2e9d4312582823f5066b3d4b7cb..bd11ee49c800df0db2a72061109aec65d5e0455e 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -35,8 +35,19 @@
       # 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.