RFR: 8325403: Add SystemGC JMH benchmarks [v4]

[Stefan Johansson]

> Please review this change to add some System GC JMH benchmarks.
> 
> **Summary**
> When developing and improving the Full GC implementation for the different GCs, using `System.gc()` to test performance is sometime very useful. This change adds a set of System GC benchmarks using JMH. The tests all have the same structure, a `@Setup` function that generates heap contents and a `@Benchmark` function which only calls `System.gc()`. The different scenarios covers some basic use-cases but more can certainly be added going forward. 
> 
> Since the benchmark function is just a single call to trigger the GC it makes sense to use the single shot JMH mode and fork a new JVM for each run. The tests also specify default heap options to avoid triggering GC during object creation to give all runs more or less the same heap layout.
> 
> **Testing**
> Local and aurora testing verifying that results are fairly stable.

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

 - Change packagename
 - Fix whitespaces

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17784/files
  - new: https://git.openjdk.org/jdk/pull/17784/files/a305e8b9..f6a2798a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17784&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17784&range=02-03

  Stats: 17 lines in 13 files changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/17784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17784/head:pull/17784

PR: https://git.openjdk.org/jdk/pull/17784