RFR: 8332082: Shenandoah: Use SATB active flag for C2 pre-write barrier on x86 and PPC

[William Kemper]

This is consistent with c1 and other platforms.

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Commit messages:
 - Check for satb active flag, rather than gc state

Changes: https://git.openjdk.org/jdk/pull/19180/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19180&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332082
  Stats: 15 lines in 2 files changed: 9 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19180.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19180/head:pull/19180

PR: https://git.openjdk.org/jdk/pull/19180