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.

Yes, the feature is currently broken, and my point is that this commit looks like it's trying to solve it (by closing https://github.com/rust-lang/rust/issues/11671). This specific commit does not close that issue as this is precisely what it's talking about.

Regardless of whether you feel it's sane or not, it's what the issue is talking about, so this commit cannot close it out.