RFR (XXS): 7142616: MAC: Honor ALT_EXPORT_PATH overrides from JDK control builds

[James Melvin]

Hi,

Typical JDK control builds override EXPORT_PATH to define where to find
freshly built HotSpot libraries. On Mac OS X, the override is broken.
Below is a solution to this problem, as well as a solution to missing
Xusage.txt files on 64-bit only builds.

This fix needs to be included in the push to master to prevent RE builds
from failing with the same problem. HotSpot and the JDK repos will build
fine separately, but fail when built in tandem such as control builds
through JPRT and RE official promotions. I'd like to request a PIT
waiver to integrate this build-only change.

WEBREV:
    http://cr.openjdk.java.net/~jmelvin/7142616/webrev.00

JPRT RUN: (internal)
 
http://prt-web.us.oracle.com//archive/2012/02/2012-02-04-212853.jmelvin.jdk7u-dev

TESTING:
    Builds permuting over 32/64-bit, manual/jprt, hotspot/jdk (or both),
      Apple/Oracle BOOTDIRs, JDK7/JDK8...
    SPECjbb2005 testsuite manual runs

Feedback welcome.

Jim