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

[Roman Kennke]

On Mon, 14 Nov 2022 07:32:25 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> This is a breakout from [JDK-8293114](https://bugs.openjdk.org/browse/JDK-8293114), which is starved for reviews. So I attempt to break up that fix into smaller units which are hopefully easier to review separately.
>> 
>> We can trim the C-heap manually using jcmd since [JDK-8268893](https://bugs.openjdk.org/browse/JDK-8268893). This patch reshapes this code, cleaning it up in an OS-agnostic way. That will allow us to add implementions for other platforms (I have this on my list for AIX at least) and make review of 8293114 easier.
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Feedback David

Looks good to me. I kinda like the OS interface. If the proliferation of lots of unimplemented methods in os/* is a concern, we could provide default impls in shared if !__GLIBC__ as a middle-ground. WDYT?

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Marked as reviewed by rkennke (Reviewer).

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