The TruncateFloat32ToUint64 operator converts a float32 to an uint64 using
round-to-zero rounding mode. If the input value is outside uint64 range, then
the result depends on the architecture. I provide an implementation for x64 and
arm64.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1479713003

Cr-Commit-Position: refs/heads/master@{#32379}