RFR(s): 8252414: Redundant suspend check when determining if a java thread is safe

[Robbin Ehn]

Hi all, please review.

In 8221207 - "Redo JDK-8218446 - SuspendAtExit hangs" we fixed so a
thread is always blocked when suspended.

And added this nice assert.
int JavaThread::java_suspend_self() {
   assert(thread_state() == _thread_blocked, "wrong state for 
java_suspend_self()");

When checking if a thread is safepoint/handshake safe there no need to 
look at ext suspend flag anymore, since the thread is blocked.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8252414
Code:
http://cr.openjdk.java.net/~rehn/8252414/webrev/index.html

Passes t1-5

Thanks, Robbin