More fixes:

- Resolve ambiguity between builtin.prebuilt and tru64.prebuilt.
- Remove archive files before creating. This is like is done for gcc,
  and removes "creating foobar.a" warnings as well.


[SVN r32886]

tools/build

@@ -1 +1 @@
-Subproject commit c0a2d3b8e6d2bc7fed220bff50a39ce98271dedf
+Subproject commit ecf5d709401f901cab7b0d7fa01995f8b41a8ce4