RFR: 8214302: Allow safely calling is_unloading() on zombie nmethods

[Erik Österlund]

Hi,

It is currently not safe to call is_unloading on zombie nmethods, unless 
it has been observed to be alive. It should be supported to make the 
code less fragile. When encountering a !is_alive() nmethod that has not 
had its unloading epoch updated, and ask if it is_unloading(), the 
answer is always false. So by adding that, is_unloading() can always be 
safely called.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8214302/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8214302

Thanks,
/Erik