[Holmake] Revise HOL_LNSIGOBJ to not "link" selftest.{uo,ui}

When running in parallel, different threads racing to link selftest
files in the sigobj directory can cause failures. Given that these
files play no useful role, we can avoid the races by simply not
moving/linking files called selftest.{uo,ui}.

tools/Holmake/Holmake_types.sml

@@ -383,11 +383,14 @@ val base_environment0 = let
                "$(patsubst %Script.sml,%Theory.uo,$(wildcard *.sml)))")]),
        ("HOLDIR", [LIT HOLDIR]),
        ("HOL_LNSIGOBJ",
-        [LIT "for i in *.uo *.ui ; do ln -fs `pwd`/",
+        [LIT "for i in `pwd`/",
+         VREF "HOLOBJDIR", LIT "/*.uo `pwd`/",
          VREF "HOLOBJDIR",
-         LIT "/$i ",
+         LIT "/*.ui ; do b=`basename $i` ; \
+             \if [ \"$b\" = \"selftest.uo\" -o \"$b\" = \"selftest.ui\" ] ; \
+             \then : ; else ln -fs $i ",
          VREF "SIGOBJ",
-         LIT " ; done && for i in *.sig ; do ln -fs `pwd`/$i ",
+         LIT " ; fi ; done && for i in *.sig ; do ln -fs `pwd`/$i ",
          VREF "SIGOBJ",
          LIT " ; echo `pwd`/`basename $i .sig` >> ",
          VREF "SIGOBJ",