Convert warning("Thread holding lock at safepoint that vm can block on") to fatal(...).
Warnings are mostly ignored during testing - tests rarely looks at VM output.
This warning may manifest about serious problem which is hard to diagnose (deadlock between VM thread & suspended Java thread).
The crash log will contain enough information to find the place where a VM lock is taken, since the check is performed on a thread which hold the lock.
Inspired by JDK-8023461.