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).

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux_nosnap_dbg,v8_linux_blink_rel
R=epertoso@chromium.org

Committed: https://crrev.com/43d3331d87f68ea7efa1b58f559f33f60e5dacfb
Cr-Commit-Position: refs/heads/master@{#35350}

Committed: https://crrev.com/253a16f690ab593a95249c34ded9df477ad9a54c
Cr-Commit-Position: refs/heads/master@{#35402}

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

Cr-Commit-Position: refs/heads/master@{#35411}