RFR: Use CardTable::card_size_in_words rather than hard coded constant [v4]

[William Kemper]

> Calculation assumed 64 words per card, which does not hold on 32 bit word platforms. The number of words per card also depends on `GCCardSizeInBytes` command line parameter.

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

  Remove stale comment

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/186/files
  - new: https://git.openjdk.org/shenandoah/pull/186/files/9ebacb5d..5a4f973d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=02-03

  Stats: 4 lines in 1 file changed: 0 ins; 3 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