Always use -Xlinker for -rpath
Currently we use -Wl which takes a list of
comma-separated options. Unfortunately that
breaks when you use it with -rpath and
a path that has commas in them.
Buck, the build system, produces paths with
commas in them.
-Xlinker doesn't have this disadvantage
and as far as I can tell is supported by
both gcc and clang. Anecdotally nvcc
supports -Xlinker, but not -Wl.
Test Plan: ./validate, harbourmaster
Reviewers: nomeata, simonmar, austin, bgamari, hvr
Reviewed By: simonmar, bgamari
Differential Revision: https://phabricator.haskell.org/D2971