RFR 8232733: Remove need to grab Threads_lock while processing handshakes

[Patricio Chilano]

Hi all,

Please review the following patch that removes the requirement of having 
to acquire the Threads_lock while processing handshakes. The guarantee 
that the JavaThread being handshaked will not exit and its memory be 
freed is already provided by using SMR ThreadsLists. With respect to the 
comments that the Threads_lock is needed to avoid a suspended JavaThread 
from being resumed, that will not be a problem since the JavaThread will 
still make the proper transition after being resumed and will block if 
needed for the handshake.

Tested with mach5, tiers1-7.

Bug: https://bugs.openjdk.java.net/browse/JDK-8232733
Webrev: http://cr.openjdk.java.net/~pchilanomate/8232733/v01/webrev/

Thanks,

Patricio