Policy issue: libc CI should not include tier 3 targets

[joshtriplett]

Currently, CI includes various tier 3 targets, and breaks if any of those targets fail. This contravenes the target tier policy, and in practice is blocking many PRs. I think we should remove these targets from CI.

[joshtriplett]

#3794

[tgross35]

Tier3 tests have been removed so I think this can be closed.