RFR: Padding used for alignment must fit an object

[William Kemper]

In some cases, alignment of new plabs on card boundaries may create padding which is too small to fill with an object. When this case is detected, we align on the plab on the next card boundary.

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Commit messages:
 - Padding used for alignment must fit an object

Changes: https://git.openjdk.java.net/shenandoah/pull/97/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=97&range=00
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/97.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/97/head:pull/97

PR: https://git.openjdk.java.net/shenandoah/pull/97