mpers.awk: add support for multidimensional arrays

mpers.awk: add support for multidimensional arrays

* mpers.awk (update_upper_bound): New function, which maintains new "count"
property for storing total element count and updates "upper_bound"
property which now contains string with array dimensions.
(/^DW_AT_upper_bound/, /^DW_AT_count/): Use it.
(what_is) <case "array_type">: Use "count" property in order to calculate
returned_size, do not embody returned string in square brackets.
* mpers_test.sh: Add checks for multidimensional arrays.