Reviewer needed: building langtools on a Mac

[Jonathan Gibbons]

Build folk,

The following fix should make it possible to (again) build langtools on 
a Mac.

Dan Smith reports:
- When building the GenStubs in OS X, the Java 6 tools classes are in 
classes.jar, and this shadows $build.bootstrap.dir, which appears in the 
classpath.  This can be fixed with -Xbootclasspath/p.

Webrev here:
    http://cr.openjdk.java.net/~jjg/7018452/webrev/

-- Jon