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.

[Firestar99]

I just rebased it and split out the fix from the test.

I started working on a difftest that would test all the various functions with wgpu, only to notice that our wgpu difftest runners are a mess and I need to clean them up first before we can use wgpu for 64bit testing. So I'm fine with keeping the ash tests as is, and I may clean them up later. (There's still #334 waiting for a review, which should speed up difftests significantly...)

Anyway, I think it's better if we land it now and take care of testing the remaining ops some other time. I may even have found an alternative way of testing them even better. Thanks for the patch ❤️