[aapcs64] Describe the FPMR register and the FP8 types (#273)

- Add a description of the FPMR register. The FPMR register
  is used to control operations with FP8 operands. It is expected
  that this register may need to be set more than once per
  function, hence it is considered a temporary register
  (clobbered by function calls).

- Add descriptions of the modal 8-bit floating point types: scalar,
   short-vector, and scalable vector. The respective parameter
   passing rules effectively state that values of these types are
   passed to and returned from functions in FP/vector registers, which
reflects the typical and anticipated place of such values in instruction
   operands.