PPC: [stubs] Don't pass name to Load/StoreGlobalViaContext stubs.

PPC: [stubs] Don't pass name to Load/StoreGlobalViaContext stubs.

Port 5dff4bdff06c0463db1e876af7541af2b715392c

Original commit message:
    No need to pass the name explicitly to the stubs; the runtime can
    extract the name from the ScopeInfo (the extension of the
    ScriptContext) on-demand easily without any performance impact.

R=bmeurer@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1258273002

Cr-Commit-Position: refs/heads/master@{#29906}