RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v3]

[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:

 - Disable TestHumongousThreshold test on generic-all for now
   
   This has experienced timeout failures on multiple platforms.
 - Cleanup code in response to reviewer feedback

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/1ae9b1c4..e094aa03

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=01-02

  Stats: 16 lines in 3 files changed: 1 ins; 4 del; 11 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