PPC: [stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

PPC: [stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

Port d6ee366d5c5aef7c6bc550889a33520058b4e33a

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.

R=bmeurer@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:510694
LOG=n

Review URL: https://codereview.chromium.org/1261473002

Cr-Commit-Position: refs/heads/master@{#29867}