RFR: 8261655: [PPC64] Build broken after JDK-8260941

[Martin Doerr]

We have to add back `CardTable* ct = ctbs->card_table();` after it was removed with JDK-8260941.

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Commit messages:
 - 8261655: [PPC64] Build broken after JDK-8260941

Changes: https://git.openjdk.java.net/jdk/pull/2548/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2548&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261655
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2548.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2548/head:pull/2548

PR: https://git.openjdk.java.net/jdk/pull/2548