RFR: 8287496: Alternative virtual thread implementation that maps to OS thread [v2]

[Alan Bateman]

> This patch adds an alternative virtual thread implementation where each virtual thread is backed by an OS thread. It doesn't scale but it can be used by ports that don't have continuations support in the VM. Aside from scalability, the lack of continuations support means:
> 
> 1. JVM TI is not supported when running with --enable-preview (the JVM TI spec allows for this) 
> 2. jshell --enable-preview can't be used (as jshell uses the debugger APIs and so needs JVM TI)
> 
> The VM option "VMContinuations" is added as an experimental option so it can be used by tests. A number of tests are changed to re-run with -XX:-VMContinuations. A new jtreg property is added so that tests that need the underlying VM support to be present can use "@requires vm.continuations" in the test description. A follow-up change would be to add "@requires vm.continuations" to the ~70 serviceability/jvmti/vthread that run with preview features enabled.

Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:

  Allowing linking without intrinsic stub, contributed-by: rehn

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8939/files
  - new: https://git.openjdk.java.net/jdk/pull/8939/files/7989708b..126e38b6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8939&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8939&range=00-01

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8939.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8939/head:pull/8939

PR: https://git.openjdk.java.net/jdk/pull/8939