[Gnu toolchain] Move GCC multilib/multiarch paths support from Linux to Gnu

Summary: The current code for GNU/Linux is actually completely generic, and can be moved to Gnu, so it can benefit GNU/Hurd and GNU/kFreeBSD

Reviewers: kristina, sammccall, lebedev.ri, MaskRay, arsenm, phosek

Reviewed By: MaskRay, phosek

Subscribers: wdng, ormris, emaste, arichardson, krytarowski, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73845