scripts: elf_helper.py: fix C++ template constexpr value processing

[pabigot]

Some DWARF symbols for members of template classes members such as numeric_limits reference are static constexpr values that do not have a data member location. Avoid attempting to dereference the value for that attribute when it isn't present.

This fix is required to get some C++ applications to link when usermode is enabled.

Below is the information for a DIE aggregate type (name, size, child) for an example:

integral_constant<bool, false> 1 DIE DW_TAG_member, size=13, has_chidren=False
    |DW_AT_name        :  AttributeValue(name='DW_AT_name', form='DW_FORM_strp', value=b'value', raw_value=23231, offset=82)
    |DW_AT_decl_file   :  AttributeValue(name='DW_AT_decl_file', form='DW_FORM_data1', value=15, raw_value=15, offset=86)
    |DW_AT_decl_line   :  AttributeValue(name='DW_AT_decl_line', form='DW_FORM_data1', value=59, raw_value=59, offset=87)
    |DW_AT_decl_column :  AttributeValue(name='DW_AT_decl_column', form='DW_FORM_data1', value=45, raw_value=45, offset=88)
    |DW_AT_type        :  AttributeValue(name='DW_AT_type', form='DW_FORM_ref4', value=35867, raw_value=35867, offset=89)
    |DW_AT_external    :  AttributeValue(name='DW_AT_external', form='DW_FORM_flag_present', value=b'', raw_value=b'', offset=93)
    |DW_AT_declaration :  AttributeValue(name='DW_AT_declaration', form='DW_FORM_flag_present', value=b'', raw_value=b'', offset=93)
    |DW_AT_const_expr  :  AttributeValue(name='DW_AT_const_expr', form='DW_FORM_flag_present', value=b'', raw_value=b'', offset=93)
    |DW_AT_inline      :  AttributeValue(name='DW_AT_inline', form='DW_FORM_data1', value=1, raw_value=1, offset=93)