workspace: Avoid using whitespace in pkg-config rpaths (RobotLocomoti…

…on#14439)

As of bazel 4.0, the user_link_flags changes from a list to a depset.

Therefore, when we used -Wl,-rpath /foo/bar as our spelling, the first
instance survived but a second instance would skips the -Wl,-rpath part
but keep the /quux/bar part, leaving a stray directory on the link line

By consolidating each rpath addition as a single list element even after
whitespace tokenization, it will remain intact.

This was biting us when fetching ipopt.pc, which has several junk
compiler paths thrown into it on Ubuntu 18.04.

tools/workspace/pkg_config.bzl

@@ -112,10 +112,10 @@ def setup_pkg_config_repository(repository_ctx):
                         linkopts[i] = linkopt
                         break
 
-        # Add `-Wl,-rpath <path>` for `-L<path>`.
+        # Add `-Wl,-rpath,<path>` for `-L<path>`.
         # See https://github.com/RobotLocomotion/drake/issues/7387#issuecomment-359952616  # noqa
         if linkopt.startswith("-L"):
-            linkopts.insert(i, "-Wl,-rpath " + linkopt[2:])
+            linkopts.insert(i, "-Wl,-rpath," + linkopt[2:])
             continue
 
         # Switches stay put.