Use more correct user agent (UA) string on ARM

[aneeshusa]

Currently, our logic to pick an appropriate default user agent string assumes that everything that isn't x86_64 is i686. However, this no longer holds true with ARM platforms also existing. We should use better values here; I'm not sure what they would be.

Files:

• components/util/opts.rs in the default_user_agent_string function

I don't think there are any tests we could write for this.

[nox]

Should we even do this?

Cc @jdm

[jdm]

Probably!