Add Interlocked{Read,Write}{8,16,Pointer} functions.
Fix the semantics for System.Threading.Volatile methods.

This is a bit tricky. The System.Threading.Thread.Volatile* methods
don't actually guarantee atomicity; all they guarantee is that the
runtime will insert acquire barriers for loads and release barriers
for stores. The methods in the System.Threading.Volatile class,
however, require both barriers *and* atomicity.

So, we need an entirely separate set of icalls for the Volatile
class in order to get the right semantics there.

These APIs are insane.
Fix mono_gc_wbarrier_generic_store_atomic semantics.

It should ensure that stores are fully atomic with barriers.