Introduce a dedicated Float64Log machine operator, that is either
implemented by a direct C call or by platform specific code, i.e.
using the FPU on x64 and ia32.

This operator is used to implement Math.log as a proper TurboFan
builtin on top of the CodeStubAssembler.

Also introduce a NumberLog simplified operator on top of Float64Log
and use that for the fast inline path of Math.log inside TurboFan
optimized code.

BUG=v8:5065

Review-Url: https://codereview.chromium.org/2029413005
Cr-Commit-Position: refs/heads/master@{#36703}