MIPS: let load and store api callbacks use global proxy as receiver.

Port r19033 (c31b3e7)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/152943002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19040 ce2b1a6d-e550-0410-aec6-3dcde31c8c00