RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v6]

[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 incrementally with one additional commit since the last revision:

  Fixes for Dan

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10123/files
  - new: https://git.openjdk.org/jdk/pull/10123/files/981a6cab..092a3acf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10123&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10123&range=04-05

  Stats: 16 lines in 7 files changed: 3 ins; 4 del; 9 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