MIPS: [stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

MIPS: [stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

Original commit message:
This is the initial round of optimizations for the
LoadGlobalViaContextStub and StoreGlobalViaContextStub, basically
turning them into platform code stubs to avoid the Crankshaft overhead
in the fast case, and making the runtime interface cheaper.

BUG=chromium:510694
LOG=n

patch from issue 1235063007 at patchset 1 (http://crrev.com/1235063007#ps1)

[paul.l..., 2015-07-20 06:11:17]

paul.lind@imgtec.com changed reviewers:
+ balazs.kilvady@imgtec.com, bmeurer@chromium.org,
dusan.milosavljevic@imgtec.com

[paul.l..., 2015-07-20 06:11:18]

PTAL, this is for inclusion in https://codereview.chromium.org/1238143002/

[balazs.kilvady, 2015-07-20 09:49:54]

l-g-t-m

[Benedikt Meurer, 2015-07-24 03:18:29]

Thanks a lot.