The Float32RoundTiesEven operator rounds float32 numbers towards the nearest
integer. If the distance to two integers is the same, then the result is
the even integer. This is the default rounding mode of the ieee 754 floating
point standard.

I implemented the optional Float32RoundTiesEven operator on x64, ia32, arm, and arm64.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1477753002

Cr-Commit-Position: refs/heads/master@{#32308}