Simplify macros for target-modifier and mitigation flags - Rebased and revised version of https://github.com/rust-lang/rust/pull/154501. --- The macros used for handling command-line flags that are “target modifiers” or “mitigations” are quite complicated, and can be significantly simplified by tweaking their syntax and by making use of `${ignore(..)}` metavars. It's possible that more code could be moved out of macros (e.g. declaring some of the enums by hand), but that can be investigated in a potential follow-up. There should be no change to compiler behaviour.