Use disassemble.c:disassembler select rl78 disassembler

Use disassemble.c:disassembler select rl78 disassembler

This patch changes rl78 to let disassble.c:disassembler select
disassembler.  rl78_get_disassembler doesn't handle the case
that abfd is NULL, so this patch also fix it.

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* rl78-tdep.c (rl78_gdbarch_init): Don't call
	set_gdbarch_print_insn.

opcodes:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* rl78-dis.c (rl78_get_disassembler): If parameter abfd
	is NULL, set cpu to E_FLAG_RL78_ANY_CPU.