RFR: Let nmethods be is_unloading() outside of safepoints

[Erik Österlund]

Hi,

All current GCs perform code cache unloading in safepoints. Therefore, 
nmethods that are alive but is_unloading() are never observed outside of 
safepoints. With concurrent class unloading, nmethods that are alive but 
is_unloading() will become observable outside of safepoints. This must 
be handled appropriately.

In this patch I changed the nmethod/compiled method iterators to accept 
parameters describing whether they should filter out not is_alive() or 
is_unloading() methods. Since there is no obvious default (all 
combinations are used depending on call site), you have to explicitly 
set what iteration mode you want.

Other than that, I make sure that the sweeper stays away from 
is_unloading() nmethods that are not yet is_unloaded(). To make the 
interactions between the sweeper and concurrent GC threads safe, I had 
to move down the store that sets the state to unloaded, and use a 
release_store there, to make sure no accesses float below it at 
compile-time. Once that store is observed, nmethods may be deleted.

In the IC miss handler, I also need to lazily clean stale IC caches due 
to calling is_unloading nmethods using nmethod entry barriers.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8213755/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213755

Thanks,
/Erik