gn files are tested on linux only.

Test:
* Built GL Swiftshader using ninja on Linux, Mac, and Windows.
* Built and ran viz_unittests on Linux, Mac, and Windows.
* Built Chromium ToT on Linux, Android, Mac.
* Built Chromium + Vulkan Swiftshader on Linux by applying
  crrev.com/c/1615710 and ran viz_unittests with --enable-vulkan

Bug: chromium:963988
Change-Id: Ic1529b8be0b508786863e03722acf610045a98ca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31529Tested-by: Saman Sami <samans@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>