MIPS: remove most uses of Isolate::Current in arch specific files.

Port r16643 (b818da5)

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/24108002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16657 ce2b1a6d-e550-0410-aec6-3dcde31c8c00