RFR: 8366781: Parallel: Include OS free memory in GC selection heuristics [v2]

[Albert Mingkun Yang]

> Add a new condition checking if OS has enough free memory to commit/expand old-gen, which determines whether the upcoming GC should be young or full.
> 
> Test: tier1-3

Albert Mingkun Yang 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:

 - review
 - Merge branch 'master' into pgc-os-free-mem
 - pgc-os-free-mem

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27068/files
  - new: https://git.openjdk.org/jdk/pull/27068/files/b435c949..f8449f31

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27068&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27068&range=00-01

  Stats: 11459 lines in 461 files changed: 6125 ins; 3001 del; 2333 mod
  Patch: https://git.openjdk.org/jdk/pull/27068.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27068/head:pull/27068

PR: https://git.openjdk.org/jdk/pull/27068