RFR: 8235696: ZGC: assert(ZNMethod::is_armed(nm) == _should_disarm_nmethods) failed: Invalid state

[Per Liden]

The assert in ZRootsIteratorCodeBlobClosure::do_code_blob() fails to 
take into account that some nmethods don't have an entry barrier at all 
and those are always considered disarmed. This patch adjusts the assert 
accordingly.

Bug: https://bugs.openjdk.java.net/browse/JDK-8235696
Webrev: http://cr.openjdk.java.net/~pliden/8235696/webrev.0

/Per