hurd: Detect libstdc++ include paths on Debian Hurd i386

This is a follow-up of e92d2b80 ("[Driver] Detect libstdc++ include
paths for native gcc (-m32 and -m64) on Debian i386") for the Debian Hurd
case, which has the same multiarch name reduction from i686 to i386.
i386-linux-gnu is actually Linux-only, so this moves the code of that commit
to Linux.cpp, and adds the same to Hurd.cpp

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D101331