Currently X87 turbofan backend use only one double register. But crankshaft needs to use more than 1
   double register. The reason is that the X87 register is stack-based and the register allocator use
   the same algorithm for all registers. So the backend of X87 needs to maintain the X87 stack status
   between basic blocks during code generation. But for Turbofan backend, it can only handle discrete
   platform-related LIRs. It's difficult to maintain the x87 stack status during different basic blocks.
   So we use the register configuration selection mechanism to return different register configuration
   for X87 turbofan and crankshaft seperately.

   This CL is a follow-up of https://codereview.chromium.org/1405673003/.

BUG=

Review URL: https://codereview.chromium.org/1413343002

Cr-Commit-Position: refs/heads/master@{#31492}