RFR: 7903425: jcstress: Pre-touch Java heap to avoid allocation stalls

[Aleksey Shipilev]

Small boards apparently timeout some allocation-hungry tests, because on-demand commit during the page allocation stalls a lot. We can make it better by automatically enabling `-XX:+AlwaysPreTouch`.

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Commit messages:
 - Do not pre-touch in sanity mode
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/134/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=134&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903425
  Stats: 26 lines in 2 files changed: 26 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jcstress/pull/134.diff
  Fetch: git fetch https://git.openjdk.org/jcstress pull/134/head:pull/134

PR: https://git.openjdk.org/jcstress/pull/134