RFR[trivial]: 8226793: Replace OopStorage dup_name with os::strdup

[Kim Barrett]

Please review this trivial change to OopStorage's name handling.
Rather than bespoke duplicatation (with dup_name unintentionally
having external linkage), it should just be using os::strdup.

CR:
https://bugs.openjdk.java.net/browse/JDK-8226793

Webrev:
http://cr.openjdk.java.net/~kbarrett/8226793/open.00/

Testing:
mach5 tier1