RFR: 8320888: Shenandoah: Enable ShenandoahVerifyOptoBarriers in debug builds

[Aleksey Shipilev]

Flag cleanup. Current barrier verification code is opt-in, and it is selected for a few tests. For extra safety, we want to have it enabled by default in debug builds. This also simplifies test configurations.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
 - [x] Linux AArch64 server fastdebug, `hotspot_gc_shenandoah`
 - [ ] Linux x86_64 server fastdebug, `tier{1,2,3,4}` with `-XX:+UseShenandoahGC`
 - [ ] Linux AArch64 server fastdebug, `tier{1,2,3,4}` with `-XX:+UseShenandoahGC`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16849/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16849&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320888
  Stats: 39 lines in 4 files changed: 0 ins; 37 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16849.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16849/head:pull/16849

PR: https://git.openjdk.org/jdk/pull/16849