RFR(xs): 8229778: TestJstatdDefaults.java failed due to "fatal error: LEAF method calling lock?"

[Robbin Ehn]

Hi all, please review.

Threads_lock is an always safepoint checked lock.
We can end up in block_if_vm_exited in all sorts of states.
But only after final safepoint started, so we just set state in vm before 
locking it.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8229778

Change-set:
http://cr.openjdk.java.net/~rehn/8229778/webrev/

Passes t1-2, x64 svc_tools and TestJstatdDefaults.

Thanks, Robbin