RFR: CODETOOLS-7902856: jcstress: Reconcile GC options

[Aleksey Shipilev]

Current GC options are questionable: the heap size too probably too large to be practical on large machines, and 4 GC threads oversubscribes the system after VM is taskset-s 2 CPUs for 2-actor tests.

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Commit messages:
 - Reconcile GC options
 - Set explicit heap size by default

Changes: https://git.openjdk.java.net/jcstress/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=15&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902856
  Stats: 54 lines in 3 files changed: 16 ins; 16 del; 22 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/15/head:pull/15

PR: https://git.openjdk.java.net/jcstress/pull/15