RFR: 8057586: Explicit GC ignored if GCLocker is active

[Ivan Walulya]

Hi All,

Please review this change to guarantee that at least a Full GC is executed between the invocation and return of `System.gc` or `WhiteBox.fullGC`, even if the call is concurrent with an active `GCLocker `. 

The change should also handle the issues reported in JDK-8299276.

Split into 3 commits, one commit for changes to each GC in [G1, Parallel, Serial]. 

Testing: Tier 1-5.

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Commit messages:
 - trailing whitespace
 - Serial Full GC
 - Parallel Full GC
 - G1 Full GC execution when JNICritical is active

Changes: https://git.openjdk.org/jdk/pull/13191/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8057586
  Stats: 335 lines in 11 files changed: 312 ins; 4 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13191/head:pull/13191

PR: https://git.openjdk.org/jdk/pull/13191