opcodes: don't assume ELF in riscv, csky, rl78, mep disassemblers

Currently, the get_disassembler() implementations for riscv, csky, and
rl78--and mep_print_insn() for mep--access ELF variants of union fields
without first checking that the bfd actually represents an ELF.  This
causes undefined behavior and crashes when disassembling non-ELF files
(the "binary" BFD, for example).  Fix that.