RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v4]

[Robbin Ehn]

> Please consider, only implemented on x64/aarch64 linux/windows. (@TheRealMDoerr have now contributed PPC64)
> 
> On my box calling clock_gettime via JNI goes from 35ns to 28ns when enabled.
> 
> Passes t1-7 with option forced on, also passes t1-4 as is in this PR.

Robbin Ehn 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 five additional commits since the last revision:

 - Merge branch 'master' into membar
 - Test fixes
 - PPC64 port courtesy Martin Doerr
 - Change header and constants handling
 - 8292591 - initial

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10123/files
  - new: https://git.openjdk.org/jdk/pull/10123/files/c33d5414..6b6ad161

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10123&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10123&range=02-03

  Stats: 62884 lines in 744 files changed: 29519 ins; 29036 del; 4329 mod
  Patch: https://git.openjdk.org/jdk/pull/10123.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10123/head:pull/10123

PR: https://git.openjdk.org/jdk/pull/10123