linker: Nub rpaths

When compiling and linking files in `ghci`, we keep adding rpath
arguments to the linker command invoation.  If those are identical we
should `nub` them out.  Otherwise we not only risk overflowing the
argument limit, but also embed huge amounts of identical rpath values
into the dynamic library, eventually leading to the overflow of the load
command size limit, due to the number of rpath entries alone.

A further improvement could be to pass `-Xlinker -dead_strip_dylibs`;
that however might be stipping too aggressively, and potentially lead to
missing symbols?

For the time being I suggest to only do the nubbing and if need be to
provide -Wl,-dead_strip_dylibs when invoking ghci.

Test Plan: ./validate

Reviewers: bgamari, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15446

Differential Revision: https://phabricator.haskell.org/D5021

(cherry picked from commit b803c406)

compiler/ghci/Linker.hs

@@ -918,16 +918,14 @@ dynLoadObjs hsc_env pls objs = do
                       -- can resolve dependencies when it loads this
                       -- library.
                       ldInputs =
-                        concatMap
-                            (\(lp, l) ->
-                                 [ Option ("-L" ++ lp)
-                                 , Option "-Xlinker"
-                                 , Option "-rpath"
-                                 , Option "-Xlinker"
-                                 , Option lp
-                                 , Option ("-l" ++  l)
-                                 ])
-                            (temp_sos pls)
+                           concatMap (\l -> [ Option ("-l" ++ l) ])
+                                     (nub $ snd <$> temp_sos pls)
+                        ++ concatMap (\lp -> [ Option ("-L" ++ lp)
+                                                    , Option "-Xlinker"
+                                                    , Option "-rpath"
+                                                    , Option "-Xlinker"
+                                                    , Option lp ])
+                                     (nub $ fst <$> temp_sos pls)
                         ++ concatMap
                              (\lp ->
                                  [ Option ("-L" ++ lp)