jmx-dev RFR: 8368527: JMX: Add an MXBeans method to query GC CPU time [v3]

[Jonas Norlinder]

> Hi all,
> 
> This PR augments the CPU time sampling measurement capabilities that a user can perform from Java code with the addition of `MemoryMXBean.getGcCpuTime()`. With this patch it will be possible for a user to measure process and GC CPU time during critical section or iterations in benchmarks to name a few. This new method complements the existing `OperatingSystemMXBean.getProcessCpuTime()` for a refined understanding.
> 
> `CollectedHeap::gc_threads_do` may operate on terminated GC threads during shutdown, but thanks to JDK-8366865 by @walulyai we can piggyback on the new `Universe::is_shutting_down`. I have implemented a stress-test `test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java` that may identify reading CPU time of terminated threads. Synchronizing on `Universe::is_shutting_down` and `Heap_lock` resolves this problem.
> 
> FWIW; To my understanding we don't want to add a `Universe::is_shutting_down` check in gc_threads_do as this may introduce a performance penalty that is unacceptable, therefore we must be careful about the few places where external users call upon gc_threads_do and may race with a terminating VM.
> 
> Tested: test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java, jdk/javax/management/mxbean hotspot/jtreg/vmTestbase/nsk/monitoring on Linux x64, Linux aarch64, Windows x64, macOS x64 and macOS aarch64 with release and fastdebug.

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

 - j.l.management with generic API docs
 - Revert "Move from j.l.management to com.sun.management etc."
   
   This reverts commit 6431310109a02ec5c34f877a1c690afb00193043.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27537/files
  - new: https://git.openjdk.org/jdk/pull/27537/files/64313101..b03db63e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27537&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27537&range=01-02

  Stats: 186 lines in 11 files changed: 35 ins; 143 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/27537.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27537/head:pull/27537

PR: https://git.openjdk.org/jdk/pull/27537