Return symbol from symbol_at_address_func

Return symbol from symbol_at_address_func

include/
	* dis-asm.h (struct disassemble_info <symbol_at_address_func>):
	Return asymbol*.
binutils/
	* objdump.c (objdump_symbol_at_address): Return asymbol*.
opcodes/
	* dis-buf.c (generic_symbol_at_address): Return symbol* NULL.
	* s12z-dis.c (decode_possible_symbol): Use symbol returned from
	symbol_at_address_func.