MIPS: thread isolate for EntryHookTrampoline.

Port r16630 (e045054)

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/23503050

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16656 ce2b1a6d-e550-0410-aec6-3dcde31c8c00