Initial crankshaft soft-float hacking.

  * modify reg allocator hooks to let half of the callee saved regs
    pretend to be FPU reg.
  * add more soft-float emulation functions in general code.
    we need to save all clobbered caller-saved regs when calling from
    crankshaft generated code into external c/c++ code.
  * initial soft-float implementation for several crankshaft hooks.
    some related testcases could go further, actually run through to
    the end, but with wrong result, needs investigation.