RFR: 8277930: Add unsafe allocation event to jfr

[xpbob]

Unsafe is used in many Java frameworks.
When the framework has a unsafe memory leak , there is no way to know what code is causing it.
Add unsafe allocation event to jfr.
Records the size and stack allocated.
This event is off by default

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Commit messages:
 - 8277930: Add unsafe allocation event to jfr

Changes: https://git.openjdk.java.net/jdk/pull/6591/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277930
  Stats: 18 lines in 4 files changed: 16 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6591.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6591/head:pull/6591

PR: https://git.openjdk.java.net/jdk/pull/6591