[jdk17u] RFR: 8273378: Shenandoah: Remove the remaining uses of os::is_MP

[Aleksey Shipilev]

Clean backport to keep codebases in sync and avoid surprises when `os::is_MP` is misbehaving.

Additional testing:
 - [x] `hotspot_gc_shenandoah` passes

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Commit messages:
 - Backport fc546d6de9a3ed33cf4b04e24e62714332b069cb

Changes: https://git.openjdk.java.net/jdk17u/pull/123/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=123&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273378
  Stats: 9 lines in 1 file changed: 6 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/123.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/123/head:pull/123

PR: https://git.openjdk.java.net/jdk17u/pull/123