RFR: JDK-8296796: Provide clean, platform-agnostic interface to C-heap trimming [v2]

[Thomas Stuefe]

On Fri, 18 Nov 2022 02:23:54 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Okay. I have some reservations about this style of approach but the precedents are there. I'd argue that for single-use situations like this and os::map_stack_shadow_pages that a XXX_ONLY(foo();) in the shared code would be acceptable. Others may disagree.
> 

Thanks David. I myself lean towards ifdefs too than having to search for the one platform implementation that actually does something. But I caught flak in the past for too many ifdefs. If it annoys us after [JDK-8293114](https://bugs.openjdk.org/browse/JDK-8293114) and no second platform implementation is forthcoming, I'll remove the APIs from shared os.

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PR: https://git.openjdk.org/jdk/pull/11089