Show error message for unknown symbol on Elf_Rel platforms

Summary: This is already the behavior on Elf_Rela platforms, and is helpful.

Reviewers: simonmar, austin, bgamari, erikd

Reviewed By: erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1841

rts/Linker.c

@@ -5009,7 +5009,6 @@ do_Elf_Rel_relocations ( ObjectCode* oc, char* ehdrC,
          } else {
             symbol = strtab + sym.st_name;
             S_tmp = lookupSymbol_( symbol );
-            if (S_tmp == NULL) return 0;
             S = (Elf_Addr)S_tmp;
          }
          if (!S) {