Replace bfd_hostptr_t with uintptr_t

Replace bfd_hostptr_t with uintptr_t

bfd_hostptr_t is defined as a type large enough to hold either a long
or a pointer.  It mostly appears in the coff backend code in casts.
include/coff/internal.h struct internal_syment and union
internal_auxent have the only uses in data structures, where
comparison with include/coff/external.h and other code reveals that
the type only needs to be large enough for a 32-bit integer or a
pointer.  That should mean replacing with uintptr_t is OK.