This CL more or less reverts commit https://codereview.chromium.org/2107733002/
The use of the MathPow code stub that was introduced by that commit caused
problems on arm64, and the MathPow code stub was also an obstacle in the
implementation of parallel code generation.

In addition this CL turns on the mjsunit/wasm/embenchen tests for arm64
which were turned off because of problems with MathPow on arm64.

R=titzer@chromium.org, bradnelson@chromium.org

Review-Url: https://codereview.chromium.org/2166793002
Cr-Commit-Position: refs/heads/master@{#37911}