Make gyp autodetection of host_arch work for 64-bit kernel and 32-bit userland

Before it would get confused and detect above situation as 64-bit, leading to build
failures. So far we've used explicit target_arch in buildbot configuration
as a workaround, but with recipes for simplicity it'd be better
if no such workaround was needed.

BUG=none
R=maruel@google.com

Review URL: https://codereview.chromium.org/151143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248214 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248214