ObjectSynchronizer::om_alloc() does not have to be public

Coleen raised a possible RFE in her review of:

JDK-8235795 replace monitor list mux{Acquire,Release}(&gListLock) with spin locks

> The function om_malloc() and many of these are 'public' in ObjectSynchronizer when they're only called within. As a new RFE, can you make them private (or even static private if possible)?
 
s/om_malloc()/om_alloc()/

Definitely makes sense to make more stuff private and, if possible,
static private. This fix (8235795) includes a bit of that, but
there's more room for improvement.