Migrate Math.imul, Math.fround, Math.acos, Math.asin and Math.atan to
C++ builtins, as these ones call into C++ anyway and so there's no
need to have this extra wrapper around it.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1731543004

Cr-Commit-Position: refs/heads/master@{#34274}