RFR: 8327097: GenShen: Align PLAB sizes down rather than up

[Kelvin Nilsen]

When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.

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Commit messages:
 - Fix disabling of TestHumongousThreshold on macosx-aarch64
 - Fix up exclude declaration for TestHumongousThreshold.java
 - Forcefully disable TestHumongousThreshold.java tier2 test
 - Try a broader exclusion for TestHumongousThreshold problem list
 - Fix two typos in Problem List
 - Disable TestHumongousThreshold.java test on macos-aarch64
 - Merge remote-tracking branch 'origin/master' into enforce-humongous-threshold-on-tlab
 - Merge branch 'openjdk:master' into master
 - Revert "Round LAB sizes down rather than up to force alignment"
 - Remove over-zealous assertion
 - ... and 8 more: https://git.openjdk.org/shenandoah/compare/fcf8a8c7...3aba4303

Changes: https://git.openjdk.org/shenandoah/pull/401/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327097
  Stats: 15 lines in 2 files changed: 4 ins; 4 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/401.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/401/head:pull/401

PR: https://git.openjdk.org/shenandoah/pull/401