The FP parameters for X87 should be put in stack and the FP return value
  should be put in stX_0 register.

  So this CL defines the FP_RETURN_REGISTERS as stX_0 only.

BUG=

Review URL: https://codereview.chromium.org/1545203002

Cr-Commit-Position: refs/heads/master@{#33110}