The TruncateFloat64ToUint64 operator converts a float64 to an uint64 using
round-to-zero rounding mode (truncate). If the input value is outside uint64
range, then the result depends on the architecture. I provide an implementation for x64 and arm64.

@v8-ppc-ports and @v8-mips-ports, can you do the implementations for ppc64 and mips64?

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1457373002

Cr-Commit-Position: refs/heads/master@{#32127}