ubsan: csky: left shift cannot be represented in type 'int'

ubsan: csky: left shift cannot be represented in type 'int'

In the following buf is an unsigned char array, so elements are
promoted to int before arithmetic operations.

  if (dis_info.info->endian == BFD_ENDIAN_BIG)
    while (n--)
      val |= buf[n] << (n*8);
  else
    for (i = 0; i < n; i++)
      val |= buf[i] << (i*8);

	* csky-dis.c (csky_chars_to_number): Rewrite.  Avoid signed
	overflow when collecting bytes of a number.