Add lint rule to catch use of #else/#elif in _api.h headers which blocks content from doxygen output

We have API.doxy defining all of our OS and arch defines in its `PREDEFINED` and are supposed to avoid `#else` or `#elif` in _api.h headers we get all content for all platforms in our docs.

I see at least one comment about this:
```
core/ir/instr_api.h:/* We resist using #elif here because otherwise doxygen will be unable to
```

But it looks like there are a bunch of instances of elses in other headers.
We should add some kind of lint rule to help us here.