hurd: Fix i386 research path

f2634184 ("[Driver] Gnu.cpp: remove obsoleted i386 triple detection
from end-of-life distribution versions") dropped the i686-gnu gcc path, but
GNU/Hurd's gcc is actually using it, and not i386.

This fixes the gcc path and update the tests to reflect it.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D101317