Expose libproc_macro in rust-src component

[kngwyu]

To use it for racer completion.
cc @nrc

[nrc]

@rust-lang/infra IS there a downside to doing this?

[kennytm]

No problem for me.

[Mark-Simulacrum]

Slightly increased file sizes, but presumably not that big a problem.

[aidanhs]

Discussed this in an infra team meeting, we don't see any issues.

[Xanewok]

It seems this was done already in #55280.