RFR: JDK-8147000: VM crashes during initialization trying to print log message

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change to fix GC logging 
during startup?

http://cr.openjdk.java.net/~brutisso/8147000/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8147000

A recent fix in hs-rt, 8146222, exposes a problem with how the 
GCId::print_prefix() method checks whether it is safe to access the 
current thread.

ThreadLocalStorage::is_initialized() does not guarantee that that 
ThreadLocalStorage::thread() will not return NULL.

Changed to use Thread::current_or_null() instead. Thanks David Holmes 
for pointing this out.

Bengt