RFR: 8296796: Provide clean, platform-agnostic interface to C-heap trimming

[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.

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Commit messages:
 - JDK-8296796-factor-out-os-trim-native-heap

Changes: https://git.openjdk.org/jdk/pull/11089/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11089&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296796
  Stats: 141 lines in 10 files changed: 89 ins; 34 del; 18 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