RFR: 8277930: Add unsafe allocation event to jfr [v5]

[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 five additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8277930
 - remove whitespace
 - add free and Reallocate event
 - 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/790cc817..b09c744d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6591&range=03-04

  Stats: 163 lines in 10 files changed: 58 ins; 83 del; 22 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