Import base::ieee754::atan() and base::ieee754::atan2() from fdlibm and
introduce Float64Atan and Float64Atan2 TurboFan operators based on those,
similar to what we already did for Float64Log and Float64Log1p. Rewrite
Math.atan() and Math.atan2() as TurboFan builtin and use the operators
to also inline Math.atan() and Math.atan2() into optimized TurboFan functions.

R=yangguo@chromium.org
BUG=v8:5086,v8:5095

Review-Url: https://codereview.chromium.org/2065503002
Cr-Commit-Position: refs/heads/master@{#36916}