Allow ignoring alignment violations

[shepmaster]

Talking with @RalfJung, they suggested that Miri could learn a flag like -Zmiri-ignore-alignment.

The broader context is that I was basically asking about suppressions (see #788), so that I could ignore errors from a dependency and see if my code introduced any additional errors. The specific errors from the dependency were alignment-related, and @RalfJung pointed out that there's the possibility of false positives.