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.

> nightly is built (with default libdir, yes), the somebody downloads and unpacks it to the folder he likes. No additional calling of install.sh or whatever happens, just copying.

This is not the only way to install rust, you are expected to use the `install.sh` script which supports itself a `--libdir` option, that is where the discrepancy arises.