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

[Aleksey Shipilev]

On Thu, 30 Sep 2021 16:45:19 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Clean backport to keep codebases in sync and avoid surprises when `os::is_MP` is misbehaving.
> 
> Additional testing:
>  - [x] `hotspot_gc_shenandoah` passes

Got the push approval.

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