[Driver] Update GCC libraries detection logic for Gentoo.

Summary:
1. Find GCC's LDPATH from the actual GCC config file.
2. Avoid picking libraries from a similar named tuple if the exact
   tuple is installed.

Reviewers: mgorny, chandlerc, thakis, rnk

Reviewed By: mgorny, rnk

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D45233

llvm-svn: 329512