Serialize execution of gcc.link.* actions when -j option is specifying.

Executing link actions in parallel hardly speeds up anything.


[SVN r27764]

tools/build

@@ -1 +1 @@
-Subproject commit b6dcad4fc717ec8eeb5ae755e95160448f6d6105
+Subproject commit f31980afe729b394cd8ace5fcd29026d99a49a9b