[Solaris] gcc toolchain handling revamp

Summary:
General idea is to utilize generic (mostly Generic_GCC) code
and get rid of Solaris-specific handling as much as possible.

In particular:
- scanLibDirForGCCTripleSolaris was removed, relying on generic
  CollectLibDirsAndTriples

- findBiarchMultilibs is now properly utilized to switch between
   m32 and m64 include & lib paths on Solaris

- C system include handling copied from Linux (bar multilib hacks)

Fixes PR24606.

Reviewers: dlj, rafael, jyknight, theraven, tstellar

Reviewed By: jyknight

Subscribers: aaron.ballman, mgorny, krytarowski, ro, joerg, cfe-commits

Differential Revision: https://reviews.llvm.org/D35755

llvm-svn: 323193