RFR: JDK-8296796: Provide clean, platform-agnostic interface to C-heap trimming [v3]

[Thomas Stuefe]

> 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into JDK-8296796-factor-out-os-trim-native-heap
 - Feedback David
 - JDK-8296796-factor-out-os-trim-native-heap

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11089/files
  - new: https://git.openjdk.org/jdk/pull/11089/files/1a642e0d..33a54c65

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11089&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11089&range=01-02

  Stats: 29294 lines in 703 files changed: 11866 ins; 14633 del; 2795 mod
  Patch: https://git.openjdk.org/jdk/pull/11089.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11089/head:pull/11089

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