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.

"it should not be done this way" is sadly not enough to stop this from happening. One of the major use cases is that we build rust on the nightly bots with one --libdir (via configure), but then it will be installed throughout various computers with other --libdir values possibly.