Re-enables ARM cross-compile.
Re-enables ARM cross-compile.

Overriding the 'product_dir' of target libdart_dependency_helper
was previously removed because of a build failure of Dartium on
Mac when building with ninja. Removal of this override caused
the ARM cross-compile to fail.

The change here achieves the same end as the 'product_dir'
override by creating two separate targets for the host and
target architectures, and works on Mac and Linux with
ninja and make.

R=iposva@google.com, kustermann@google.com

Review URL: https://codereview.chromium.org//14359014

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@22139 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk