make: rustdoc failures can leave around expected build product

[pnkfelix]

rustdoc failures can leave around expected build product, e.g. doc/syntax/index.html.

This means that after the first make attempt fails, if you invoke make again, it will see the build product now in place, and will continue as if the dependency has been satisified.

This is irritating if you are trying to understand how the first error happened in the first place and want to find out e.g. what the necessary command line invocation to rustdoc is needed to reproduce it.

I would recommend we change the makefiles to use an analogous stamp.syntax infrastructure for the docs that we already use for the libraries (and then you would only touch the stamp file after rustdoc had finished generating its output and returned without an error).

[pnkfelix]

part of #8058

[steveklabnik]

Triage: no change. This can still happen, as we haven't changed to using stamps.

[steveklabnik]

Triage: I think rustbuild Just Fixes this. @alexcrichton ?

[alexcrichton]

Yes I think because we're now leveraging cargo doc this is just taken care of.