compiletest: consider transitioning `//@ only-*` target-based directives to e.g. `//@ only-target_arch: arch1 arch2`

Which allows permitting e.g. both `aarch64` + `x86_64`. Currently

```
//@ only-x86_64
//@ only-aarch64
```

means the test will never run, since this is AND of `x86_64` + `aarch64` target archs.

Also might want to consider detecting "test will never run" directive combos.

Blocked: don't pile on hacks, name-value directive parsing in compiletest desperately needs cleanup.