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.

Note `if libdir != "lib"`, so env variable will be used only if it was set to something not default. And if it was set, user defenetely wants rustc use it, not the fallback.