If the architecture does not provide rounding instructions, then C
implementations of these rounding instructions are called. The C
implementations from math.h are used, function pointers are registered
as external references so that they can be call from the simulator.

R=titzer@chromium.org

BUG=575379

LOG=Y

Review URL: https://codereview.chromium.org/1661463002

Cr-Commit-Position: refs/heads/master@{#33677}