The Float32RoundTruncate operator rounds float32 numbers towards zero.
The operator is currently implemented on x64, ia32, arm, and arm64.

Additionally I added support for the float32 vrintz, vrintn, and vrinta
instructions to the arm simulator.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1468303005

Cr-Commit-Position: refs/heads/master@{#32301}