[Driver] Wire up the -f[no-]rtlib-add-rpath flag and tests

D30700 added the -f[no-]rtlib-add-rpath flag, but that flag was never
wired up in the driver and tests were updated to check whether it
actually does anything. This patch wires up the flag and updates test.

Differential Revision: https://reviews.llvm.org/D45145

llvm-svn: 329032