Fix build of new SYMBOL-INFO magic on targets without compare-and-swap-vops.

  * The (non-atomic) fallback position for compare-and-swap
requires some way to set the target value, which can be provided
as an explicit function name or will default to a SETF function.

  * Define (SETF SYMBOL-INFO) as a :SET-TRANS for the SYMBOL-INFO
slot and provide an interpreter stub for it.