RFR: 8277137: Set OnSpinWaitInst default value to "isb" for Arm Neoverse N1

[Evgeny Astigeevich]

One `ISB` implementation of `Thread.OnSpinWait` shows performance improvements on Graviton2 (Arm Neoverse N1 implementation), e.g. https://github.com/openjdk/jdk/pull/5562#issuecomment-966153163. 

Testing:
- `make test TEST=gtest`: Passed
- `make run-test TEST=tier1`: Passed
- `make run-test TEST=tier2`: Passed
- `make run-test TEST=hotspot/jtreg/compiler/onSpinWait`: Passed

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Commit messages:
 - 8277137: Set OnSpinWaitInst default value to "isb" for Arm Neoverse N1

Changes: https://git.openjdk.java.net/jdk/pull/6415/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6415&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277137
  Stats: 100 lines in 2 files changed: 100 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6415.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6415/head:pull/6415

PR: https://git.openjdk.java.net/jdk/pull/6415