RFR: 8277503: compiler/onSpinWait/TestOnSpinWaitAArch64DefaultFlags.java failed with "OnSpinWaitInst with the expected value isb not found."

[Evgeny Astigeevich]

OnSpinWaitInst/OnSpinWaitInstCount are diagnostic options.
'XX:+PrintFlagsFinal' does not print diagnostic options in release builds which causes the test failure. 
The fix is to use '-XX:+UnlockDiagnosticVMOptions'.

Testing: release and fastdebug builds
- `make test TEST=hotspot/jtreg/compiler/onSpinWait/TestOnSpinWaitAArch64DefaultFlags.java`: Passed

-------------

Commit messages:
 - 8277503: compiler/onSpinWait/TestOnSpinWaitAArch64DefaultFlags.java failed with "OnSpinWaitInst with the expected value isb not found."

Changes: https://git.openjdk.java.net/jdk/pull/6521/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6521&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277503
  Stats: 4 lines in 2 files changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6521.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6521/head:pull/6521

PR: https://git.openjdk.java.net/jdk/pull/6521