RFR: 8256832: Zero: micro-optimize safepoint handling after JDK-8255384

[Aleksey Shipilev]

Now that [JDK-8255384](https: //bugs.openjdk.java.net/browse/JDK-8255384) effectively made the safepoints conditional for Zero, we can do `HandleMarkCleaner` under the safepoint check. This seems to improve Linux x86_64 Zero release `make bootcycle-images`: from 33.0m to 32.5m.

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Commit messages:
 - 8256832: Zero: micro-optimize safepoint handling after JDK-8255384

Changes: https://git.openjdk.java.net/jdk/pull/1378/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1378&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256832
  Stats: 9 lines in 1 file changed: 0 ins; 3 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1378.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1378/head:pull/1378

PR: https://git.openjdk.java.net/jdk/pull/1378