RFR (XS) 8199012: JVM crash during LevelDB JNI access

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8199012/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8199012

JNI DetachCurrentThread is defined as a no-op if called by a thread that 
is not attached. But the check for that was occurring after we called 
VM_Exit::block_if_vm_exited(). If the thread was not in fact attached 
then we have a NULL current thread and that can cause in crash in 
VM_Exit::block_if_vm_exited() if we actually need to block - which 
explains the scenario reported in the bug.

Simple, obvious, fix is to do the "not attached" check first.

Testing:
   - Mach5 CI equivalent testing (tier1/2)
   - internal JNI tests

I marked this noreg-hard as a regression test is difficult to set up due 
to the need to coordinate with a VM shutdown.

Thanks,
David