[jdk17u-dev] RFR: 8339687: Rearrange reachabilityFence()s in jdk.test.lib.util.ForceGC

[Goetz Lindenmaier]

I backport this for parity with 17.0.15-oracle.

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Commit messages:
 - Backport 51b85a1f692fed7a66bdc0fae21438a60aafe7c2

Changes: https://git.openjdk.org/jdk17u-dev/pull/3125/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3125&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339687
  Stats: 22 lines in 1 file changed: 3 ins; 2 del; 17 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3125.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3125/head:pull/3125

PR: https://git.openjdk.org/jdk17u-dev/pull/3125