Add the rustdoc_ng binary to the makefile rules

Now rustdoc_ng will be built as both a binary and a library (using the same
rules as all the other binaries that rust has). Furthermore, this will also
start building rustdoc_ng unit tests (and running them).
auto merge of #9241 : alexcrichton/rust/build-rustdoc-ng, r=catamorphism

Now rustdoc_ng will be built as both a binary and a library (using the same
rules as all the other binaries that rust has). Furthermore, this will also
start building rustdoc_ng unit tests (and running them).

Note that some `rustdoc_ng` tests were removed, but @cmr says they weren't supposed to be there in the first place. The rustdoc_ng code should also be included in `make install` and `make dist` now.