Make these builtins fast by default w/o relying on the
%_StringCharCodeAt and %_StringCharAt intrinsics, which we cannot deal
with well in TurboFan (and ignition).

R=epertoso@chromium.org

Review URL: https://codereview.chromium.org/1868963002

Cr-Commit-Position: refs/heads/master@{#35350}