port 74434403 (r32261)

  original commit message:
  I implemented the optional Float32RoundDown operator on x64, ia32, arm,
  and arm64.

  For arm I also had to adjust the simulator.

BUG=

Review URL: https://codereview.chromium.org/1490113003

Cr-Commit-Position: refs/heads/master@{#32492}