[M68k] Basic FP register files and FMOVE implementations

  - FP registers and register classes definitions
  - MC support for FP registers
  - MC support for the FPDR -> FPDR variant of FMOVE/FSMOVE/FDSMOVE

Note that we don't implement 32/64-bit FP registers with subregisters since
f32/f64 values will be converted to f80 upon storing into a register,
rather than occupying part of the register.
So we only use register classes with different value types to distinguish
them.

Differential Revision: https://reviews.llvm.org/D147480