Look for crates in LIBDIR, provided by --libdir option

Fixies #11671

This commit changes default relative libdir 'lib' to a relative libdir calculated using LIBDIR provided by --libdir configuration option. In case if no option was provided behavior does not change.

The reasoning is the same as [this comment](https://github.com/rust-lang/rust/pull/19065#discussion_r20627950) I believe (I'll reply in a just a bit).