gas md_apply_fix bad casts

ns32k and z8k cast a valueT pointer to a long pointer when loading
md_apply_fix's value.  That's quite wrong if the types have different
sizes, as they may eg. on a 32-bit host with 64-bit bfd support.
sparc also loads the value via a cast pointer, but at least in that
case the cast is to the same size pointer.  None of these casts are
needed.  Get rid of them.