We implement the conversions via C functions (external references)
because 64-bit conversions are difficult to implement on ia32 and
would not be significantly faster.

Bug: v8:10520
Change-Id: I3573d30bf6b3d558f708c7e0273f027c30a1a771
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202988
Commit-Queue: Richard Stotz <rstz@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67883}