6814943: getcpool001 catches more than one JvmtiThreadState problem
Summary: Mark field volatile, use membars, and change access order to close race
Reviewed-by: dcubed, dholmes
6458402: 3 jvmti tests fail with CMS and +ExplicitGCInvokesConcurrent
Summary: Make JvmtiGCMark safe to run non-safepoint and instrument CMS
Reviewed-by: ysr, dcubed