RFR: 8303210: [linux, Windows] Make UseSystemMemoryBarrier available as product flag [v6]

[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.
> 
> Update: Startup performance and some benchmarks on some platforms are impacted (see below). So, this PR no longer switches it on by default.

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

 - Remove unintended newline from os_aix.cpp.
 - Remove leftover from os_aix.cpp.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12753/files
  - new: https://git.openjdk.org/jdk/pull/12753/files/5fc7dbd6..adcc0cd6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12753&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12753&range=04-05

  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12753.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12753/head:pull/12753

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