[Driver] Add flag to request arch-specific-subdir in -rpath

Summary:
This patch adds -f[no-]rtlib-add-rpath, which if enabled, embeds the
arch-specific subdirectory in resource directory using -rpath (instead
of doing so only during native compilation).

This patch also re-enables test arch-specific-libdir.c which was
silently unsupported because of the REQUIRES tag 'linux'.

Reviewers: bkramer, rnk, mgorny

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D30700

llvm-svn: 297751