[Driver] Fix arch-specific-libdir-rpath.c

Summary: Fix the test by adding missing -target flags with a 'linux' triple.

Reviewers: rnk, srhines

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30947

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297754 91177308-0d34-0410-b5e6-96231b3b80d8