Add better description of the output of x87-floating-point-modes

;;

;; When one of the mask bits (0-5) is set, then that exception is

;; masked so that no exception is generated.

;;

;; Returns the control and status words merged into one.  The low 16

;; bits contains the control word with the exception mask bits

;; inverted to indicate exception enable bits.  The high 16 bits

;; contains the status word, but the top 8 bits of the status word are

;; cleared, effectively removing the condition code, top-of-stack

;; bits, and the FPU busy bit.

(define-vop (x87-floating-point-modes)

  (:results (res :scs (unsigned-reg)))

  (:result-types unsigned-num)