DWARF-5: DW_FORM_data16
    
    DWARF-5 has new form DW_FORM_data16.  The problem is that GDB cannot pass
    16-byte constant as a constant value as that would require GDB to use GCC
    extension __int128.
    
    Formerly such data was coded as DW_FORM_block* so GDB still decodes
    DW_FORM_data16 like DW_FORM_block*.
    
    gdb/ChangeLog
    2017-02-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
    
    	* dwarf2read.c (skip_one_die, read_attribute_value)
    	(dwarf2_const_value_attr, dump_die_shallow)
    	(dwarf2_get_attr_constant_value, dwarf2_fetch_constant_bytes)
    	(skip_form_bytes, attr_form_is_constant): Handle DW_FORM_data16.
    
    gdb/testsuite/ChangeLog
    2017-02-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
    
    	* gdb.dwarf2/formdata16.c: New file.
    	* gdb.dwarf2/formdata16.exp: New file.
    	* lib/dwarf.exp (Dwarf): Add DW_FORM_data16.