MIPS64:  Reland r22082 "Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

Port r22129 (676bb14).

TEST=
BUG=
R=balazs.kilvady@imgtec.com, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/395803002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22410 ce2b1a6d-e550-0410-aec6-3dcde31c8c00