[jdk25u] RFR: 8359947: GenShen: use smaller TLABs by default

[William Kemper]

Clean backport. This is a performance improvement for the generational mode of Shenandoah.

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Commit messages:
 - Backport a0c3efa6a80fe95e22faf5a732e42f1e1291fb4c

Changes: https://git.openjdk.org/jdk25u/pull/99/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=99&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359947
  Stats: 13 lines in 3 files changed: 7 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk25u/pull/99.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/99/head:pull/99

PR: https://git.openjdk.org/jdk25u/pull/99