rustbuild: Build documentation for `proc_macro`

This commit fixes #38749 by building documentation for the `proc_macro` crate by
default for configured hosts. Unfortunately did not turn out to be a trivial
fix. Currently rustbuild generates documentation into multiple locations: one
for std, one for test, and one for rustc. The initial fix for this issue simply
actually executed `cargo doc -p proc_macro` which was otherwise completely
elided before.

Unfortunately rustbuild was the left to merge two documentation trees together.
One for the standard library and one for the rustc tree (which only had docs for
the `proc_macro` crate). Rustdoc itself knows how to merge documentation files
(specifically around search indexes, etc) but rustbuild was unaware of this, so
an initial fix ended up destroying the sidebar and the search bar from the
libstd docs.

To solve this issue the method of documentation has been tweaked slightly in
rustbuild. The build system will not use symlinks (or directory junctions on
Windows) to generate all documentation into the same location initially. This'll
rely on rustdoc's logic to weave together all the output and ensure that it ends
up all consistent.

Closes #38749

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