RFR: 8361520: Stabilize SystemGC benchmarks [v2]

[Aleksey Shipilev]

> Noticed this while working on a related bug ([JDK-8359960](https://bugs.openjdk.org/browse/JDK-8359960)):
> 
> First, I see the benchmark executes a single shot per fork. As such, I believe the benchmark really tests the cost of initial GC, that probably drags a lot of (potentially non-benchmark-related) objects through new (possibly awkwardly wired, despite +AlwaysPreTouch) memory. The first iteration is 80 ms/op for me here, and the second one is -- whoosh -- only 3 ms/op! Second, the benchmark is really, really noisy. Part of it is due to first iteration being noisy, but also we want more samples to shrink the estimated errors.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `gc.systemgc` benchmark runs

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Revert accidental comment removal

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26182/files
  - new: https://git.openjdk.org/jdk/pull/26182/files/5e22da78..1e524119

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26182&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26182&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26182.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26182/head:pull/26182

PR: https://git.openjdk.org/jdk/pull/26182