RFR: Age objects before installing forwarding pointer to them

[William Kemper]

We cannot safely manipulate the mark word concurrently with mutators. This _should_ fix a crash issue reported on PPC 64.

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Commit messages:
 - Age objects before installing forwarding pointer to them

Changes: https://git.openjdk.java.net/shenandoah/pull/122/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=122&range=00
  Stats: 14 lines in 1 file changed: 4 ins; 8 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/122.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/122/head:pull/122

PR: https://git.openjdk.java.net/shenandoah/pull/122