Support callee save floating point registers on x64.

- Share the computation of core_spill_mask and fpu_spill_mask
  between backends.
- Remove explicit stack overflow check support: we need to adjust
  them and since they are not tested, they will easily bitrot.

Change-Id: I0b619b8de4e1bdb169ea1ae7c6ede8df0d65837a
11 files changed