RFR: 8303210: [linux, Windows] Enable UseSystemMemoryBarrier by default if possible [v4]

[Martin Doerr]

> I'd like to enable UseSystemMemoryBarrier by default on supported Operating Systems in order to improve performance of thread state transitions (I/O, JNI, foreign function calls, JIT compiler threads, etc.). See JBS issue for more details.
> Unfortunately, the feature was not yet implemented on all platforms. I added the code, but need the platform maintainers to check if it can be used reliably (and ideally if the performance improves). It's easy to switch it off again in case of problems.

Martin Doerr has updated the pull request incrementally with two additional commits since the last revision:

 - Make it a regular product flag and keep it off by default.
 - Add includes for riscv.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12753/files
  - new: https://git.openjdk.org/jdk/pull/12753/files/4660291f..b7278798

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12753&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12753&range=02-03

  Stats: 3 lines in 3 files changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12753.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12753/head:pull/12753

PR: https://git.openjdk.org/jdk/pull/12753