This patch refactors the Atomics builtins so that they are implemented
as C++ builtins rather than experimental JS builtins. Previously, each
of these functions called out to a runtime function, so no significant
change in performance is anticipated. The goal of this patch is to
remove the last user of experimental JS builtins so that the mechanism
can be removed, for performance reasons. The patch includes a drive-by
fix of a check-fail. For the most part, the patch is just moving code
without modification from runtime-atomics.cc to
builtins-sharedarraybuffer.cc .

BUG=v8:5880

Review-Url: https://codereview.chromium.org/2698813004
Cr-Commit-Position: refs/heads/master@{#43335}