ANGLE already has this logic, but with Dawn looking to use Swiftshader
as well, there would have been a conflict when both ANGLE and Dawn tried
to generate the same ICD JSON file.

Having this logic in Swiftshader's GN files means that any number of
component of projects using GN (aka Chromium) can use swiftshader
without issues.

Bug: dawn:283
Change-Id: I8531c98c370db01563dc102f259164e0393242ca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38491
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Corentin Wallez <cwallez@google.com>