RFR 8242484: Rework thread deletion during VM termination

[Patricio Chilano]

Hi,

Please review this small follow up for 8240918 to always delete the 
JavaThread that calls Threads::destroy_vm().
In 8240918 I added a conditional check in Threads::destroy_vm() to 
prevent deleting a JavaThread when there are handshakers blocked in its 
handshake_turn_sem semaphore.  We can actually avoid this issue 
altogether if after removing the JavaThread from the active list and 
before triggering the terminating safepoint, we wait until the 
JavaThread is no longer protected by a ThreadsList reference.

Tested in mach5 tiers 1-6.

Bug:
    https://bugs.openjdk.java.net/browse/JDK-8242484

Webrev:
    http://cr.openjdk.java.net/~pchilanomate/8242484/v1/webrev/

Thanks,
Patricio