RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v2]

[Kelvin Nilsen]

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

Kelvin Nilsen has updated the pull request incrementally with two additional commits since the last revision:

 - Add comment to explain requirement for PLAB alignment
 - Assure PLAB alignment by construction rather than fixup

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/3aba4303..1ae9b1c4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=00-01

  Stats: 39 lines in 3 files changed: 27 ins; 5 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