RFR: 8305425: Thread.isAlive0 doesn't need to call into the VM [v6]

[David Holmes]

> We have the strange situation where calling `t.isAlive()` on a `java.lang.Thread` `t`, will call into the VM (via `alive()` then `isAlive0()`) where the VM then examines the `eetop` field of `t` to extract its `JavaThread` pointer and compare it to null. We can simply read `eetop` directly in `Thread.alive()`:
> 
> boolean alive() {
>   return eetop != 0;
> } 
> 
> I also updated a comment in relation to `eetop`.
> 
> Testing: tiers 1-3
> 
> Thanks

David Holmes has updated the pull request incrementally with two additional commits since the last revision:

 - Switch from using synchronized to using a volatile eetop field
 - Added Shipilev's test (with a small addition)

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13287/files
  - new: https://git.openjdk.org/jdk/pull/13287/files/10022a06..d284547c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13287&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13287&range=04-05

  Stats: 122 lines in 3 files changed: 116 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13287.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13287/head:pull/13287

PR: https://git.openjdk.org/jdk/pull/13287