RFR: Explicitely convert const oopDesc* to oop in typeArrayOop barriers

[Roman Kennke]

My last patch to typeArrayOop.hpp introduced a build failure in release
builds. It requires an explicit conversion from const oopDesc* to oop to
pass into the write-barrier.

http://cr.openjdk.java.net/~rkennke/conv-oop/webrev.00/

Ok?

Roman