RFR: 8211804: Constant AO_UNUSED_MBZ uses left shift of negative value

[Kim Barrett]

Please review this small change to native code in the (deprecated)
jdk.pack module.  The existing code triggers a warning when building
with gcc with -std=c++11 or -std=c++14.

CR:
https://bugs.openjdk.java.net/browse/JDK-8211804

Webrev:
http://cr.openjdk.java.net/~kbarrett/8211804/open.00/

Testing:
Mach5 tier1.