Avoid loading needless proc-macro dependencies

Fixes #37958 when no proc-macros are exported; in particular, without `pub extern crate proc_macros;`, `#![feature(macro_reexport)]`, or `#![feature(use_extern_macros)]`.

I opened rust-lang/cargo#3334 for exported proc macros.

r? @alexcrichton

Now that proc-macro crates can reexport from their dependencies we need to be
able to find the other crates, so ensure that we pass an appropriate -L flag.

Closes #3334

Now that proc-macro crates can reexport from their dependencies we need to be
able to find the other crates, so ensure that we pass an appropriate -L flag.

Closes #3334

Add host dependency path via -L for cross compiles

Now that proc-macro crates can reexport from their dependencies we need to be
able to find the other crates, so ensure that we pass an appropriate -L flag.

Closes #3334