After the refactoring for supporting f32 conversions, adding f64
conversions is straight forward.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I8f76fbef3f1e67ecdb02dd1d103868f2d1d3226f
Reviewed-on: https://chromium-review.googlesource.com/957026
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51864}