RFR: Use CardTable::card_size_in_words rather than hard coded constant [v2]

[William Kemper]

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

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Fix warning

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/186/files
  - new: https://git.openjdk.org/shenandoah/pull/186/files/0d87f14a..521c1491

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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