compiler: sort packages in export data more deterministically

We can have multiple packages with the same name, so also sort by pkgpath.
    To avoid an inconsistent sort, sort by symbol and pointer address if
    we somehow get two different packages with the same name and pkgpath.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/171032

From-SVN: r270220

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-a69f7c05f1880bb90544fb0c3577109cb1d7f3ab
+8822487ed776d55eafed44de7d89ee54bbfbab47
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/export.cc

@@ -509,7 +509,24 @@ Export::set_type_index(Type* type)
 static bool
 packages_compare(const Package* a, const Package* b)
 {
-  return a->package_name() < b->package_name();
+  if (a->package_name() < b->package_name())
+    return true;
+  else if (a->package_name() > b->package_name())
+    return false;
+
+  if (a->pkgpath() < b->pkgpath())
+    return true;
+  else if (a->pkgpath() > b->pkgpath())
+    return false;
+
+  // In principle if we get here then a == b.  Try to do something sensible
+  // even if the import information is inconsistent.
+  if (a->pkgpath_symbol() < b->pkgpath_symbol())
+    return true;
+  else if (a->pkgpath_symbol() > b->pkgpath_symbol())
+    return false;
+
+  return a < b;
 }
 
 // Write out all the known packages whose pkgpath symbol is not a