Look for standard crates in LIBDIR provided by --libdir option,
not in hardcoded libdir path. If there was no LIBDIR provided
during configuration fallback to hardcoded paths.

Thanks to Jan Niklas Hasse for solution and to Alex Crichton for improvements.

Closes #11671
auto merge of #16552 : jauhien/rust/fix-libdir, r=alexcrichton

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.