Turn off SupportsFlexibleFloorAndRound for Arm64 due to a bug.

There is a bug with SupportsFlexibleFloorAndRound on Arm64. Turn it off for
now until we figure out a proper solution.

BUG=chromium:476477
LOG=Y

Review URL: https://codereview.chromium.org/1093863002

Cr-Commit-Position: refs/heads/master@{#27925}