Fix error messages in the NFP sources when building for 32-bit targets.

bfd	* targets.c: Wrap nfp_elf64_vec in BFD64 ifdef.

include	* opcode/nfp.h: Use uint64_t instead of bfd_vma.

opcodes	* nfp-dis.c: Use uint64_t for instruction variables, not bfd_vma.