This change fixes some bugs in the ppc64 port:
- temp reg r0 overwritten in decode_klass_not_null()
- check for full code cache was missing in some places
- decodeNKlass had edge of type RegN which got an oop_map and broke GC.
And some other improvements
- remove some superfluous isync instructions
- set SUPPORTS_NATIVE_CX8
- implement some new Power8 instructions to be used later