RFR: Use CardTable::card_size_in_words rather than hard coded constant

[William Kemper]

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

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Commit messages:
 - Use CardTable::card_size_in_words rather than hard coded constant
 - Add more detail to assertion message

Changes: https://git.openjdk.org/shenandoah/pull/186/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=186&range=00
  Stats: 17 lines in 2 files changed: 6 ins; 4 del; 7 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