RFR(M): 8068503: ppc64: Encode/Decode nodes for disjoint cOops mode

[Lindenmaier, Goetz]

Hi,

this change contains the implementation (activation) of the
encode/decode nodes exploiting the disjoint heap base
available since "8064457: Introduce compressed oops mode disjoint base..."

Please review this change.
http://cr.openjdk.java.net/~goetz/webrevs/8068503-ppcDisjoint/

This is ppc64-only, so I don't need a sponsor ;)

Best regards,
  Goetz.