This implement float to i64/u64 conversions on ia32 and x64.
These conversions emit a C call on ia32, and are implemented using
native instructions on x64.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I5b97a74d336e196598b29d407a3d06405b74ee14
Reviewed-on: https://chromium-review.googlesource.com/1014114
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52705}