x/tools/cmd/guru: extend 'definition' to understand go:linkname and assembly

[mdempsky]

This is a bit of a ponies request, but it would be really convenient while working on the standard library if "guru definition" could handle go:linkname and assembly. Here's how I would suggest this work:

1. If definition resolves an identifier to a non-function declaration or a function declaration with a function body, do the same thing as now.
2. If there's a go:linkname comment in any of the .go files that references the function name, then recursively look for the remote name's definition.
3. Search the package's .s files for a TEXT declaration that declares the function symbol; if found, return that location.
4. If the function declaration is within $GOROOT, then apply steps 2 and 3 but searching within package runtime instead.
5. Else, return the body-less function declaration's position.

The order of steps 2 and 3 aren't critical.

Step 4 is clunky, but necessary for the standard repo. I think outside of the standard library steps 2 and 3 alone should suffice.

/cc @alandonovan

[adonovan]

guru was deleted in #65880; closing as obsolete. Incidentally, gopls understands linkname (though not assembly, which would indeed be nice).