Previously all contexts had a link to the global object, but what is
required in most cases (except for the global load, store and delete
case) is the native context.

This also removes the second dummy global object that was still linked
to every native context. We will add a different mechanism to ensure
that builtins do not pollute the actual global object during
bootstrapping.

Drive-by-fix: Unify some MacroAssembler magic and drop obsolete stuff.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1480003002

Cr-Commit-Position: refs/heads/master@{#32375}