mk: Add a dummy CFG_COMPILER_HOST_TRIPLE to rustdoc invocation.

Otherwise it will prohibit `make compiler-docs` on Windows.
rustdoc: Clean the `initSearch` routine up.
rustdoc: Prune the paths that do not appear in the index.

For the full library and compiler docs, the size of
`search-index.js` decreases by 13% (18.5% after gzip -9)
which is a substantial gain.
auto merge of #13402 : lifthrasiir/rust/rustdoc-prune-paths, r=alexcrichton

`allPaths` entries in `search-index.js` are only meaningful when there are `searchIndex` items that refer to them. Pruning unused paths gives a substantial gain in the search index size, especially when both the library docs and compiler docs are generated altogether:

```
 1719473 search-index-old.js
 1503299 search-index.js (13% gain)

  262711 search-index-old.js.gz
  214205 search-index.js.gz (18.5% gain)
```

...where `*.gz` files are gzipped with `gzip -9`. This commit also cleans the `initSearch` JavaScript routine a bit.