Added tests for 64bit trailing_zeros

[voxelbee]

Fix for issue #501

[Firestar99]

1. Looks like swiftshader doesn't support int64... I was looking into switching to lavapipe anyway for unrelated reasons, guess I got to finish that one.

2. wgpu has a native-only feature called SHADER_INT64, so you don't necessarily need to use the ash runner

3. I wouldn't be surprised if there's more integer intrinsics that have issues. These in particular have been added before we had difftests, and I wanted to some for quite a while already.