RFR: 8277930: Add unsafe allocation event to jfr [v9]

[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

xpbob has updated the pull request incrementally with two additional commits since the last revision:

 - Merge branch 'JDK-8277930' of https://github.com/xpbob/jdk into JDK-8277930
 - add threshold

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6591/files
  - new: https://git.openjdk.java.net/jdk/pull/6591/files/bf3a9c28..4fb24134

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=07-08

  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 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