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

[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

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

Commit messages:
 - update symbol file
 - 8305425: Thread.isAlive0 doesn't need to call into the VM

Changes: https://git.openjdk.org/jdk/pull/13287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305425
  Stats: 19 lines in 5 files changed: 3 ins; 12 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