Port 47502a23

Original commit message:
    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=bmeurer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1491433002

Cr-Commit-Position: refs/heads/master@{#32435}