Enable setting rpath on install

[taralx]

For the (fairly common?) case of setting a custom prefix and nothing else, it would be useful to set rpath during install. Mac OS specifically has "install_name_tool" to manage this process. Thoughts?

(Forked from #20836.)

[steveklabnik]

/cc @brson @Diggsey @edunham , who have been working on a new installer

[brson]

I think I'm in favor. Would need to be added to rust-installer probably, which already has an optional ldconfig post-install step on linux. We've discussed elsewhere that the rpaths baked into rustc on OS X are broken, and a solution would be to frob them during install.

[edunham]

It's now on by default.