Allow gyp to generate 64 bit project files for VS2012.
Allow gyp to generate 64 bit project files for VS2012.

Visual Studio 2010 express was the last version to not support 64 bit
configurations.

BUG=angle:714

Change-Id: Ie4383a437a98cecfc82702e766a460ade96062f8
Reviewed-on: https://chromium-review.googlesource.com/210824
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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