In CL 0fe2fbd1 the implementation of
  MathPow for all ports are unified and MathPow stub code is invoked.
  So we move the direct runtime function call from full-codegen to MathPow
  stub for X87 platform.

BUG=

Review URL: https://codereview.chromium.org/1258873002

Cr-Commit-Position: refs/heads/master@{#29865}