RFR: Use CardTable::card_size_in_words rather than hard coded constant [v3]

[William Kemper]

> Calculation assumed 64 words per card, which does not hold on 32 bit word platforms

William Kemper has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Do not assume cards will have 64 words

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/186/files
  - new: https://git.openjdk.org/shenandoah/pull/186/files/521c1491..9ebacb5d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=01-02

  Stats: 20 lines in 2 files changed: 5 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/shenandoah/pull/186.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/186/head:pull/186

PR: https://git.openjdk.org/shenandoah/pull/186