RFR: 8277930: Add unsafe allocation event to jfr [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8277930
 - 8277930: Add unsafe allocation event to jfr

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6591/files
  - new: https://git.openjdk.java.net/jdk/pull/6591/files/a30f3618..d883f62d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=00-01

  Stats: 450 lines in 28 files changed: 164 ins; 169 del; 117 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