RFR(s): 8235913: ThreadStop should be a handshake

[Robbin Ehn]

Hi all, please review:

Making VM_ThreadStop into a handshake instead.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8235913

Code:
http://cr.openjdk.java.net/~rehn/8235913/v1/webrev/index.html

Passes t1-7.

Thanks, Robbin