RFR: JDK-8287671: Adjust ForceGC to invoke System::gc fewer times for negative case

[Mandy Chung]

This reapplies JDK-8287384 and adjust the number of GCs for negative case, i.e. the condition is never met that is used to verify a reference is not GC'ed.

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Commit messages:
 - JDK-8287671: Adjust ForceGC to invoke System::gc fewer times for negative case

Changes: https://git.openjdk.java.net/jdk/pull/9021/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9021&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287671
  Stats: 19 lines in 1 file changed: 11 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9021.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9021/head:pull/9021

PR: https://git.openjdk.java.net/jdk/pull/9021