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.

Why not just always ensure CFG_LIBDIR_RELATIVE is set an use it? This allows us to move the defaults out of rustc and keep them all in configure.