DWARF: Emit DWARF5 forms for indirect addresses and string offsets.
We already emit DWARF5 attributes and tables for indirect addresses
and string offsets, but still use GNU forms. Add a new helper function
dwarf_FORM () for emitting the right form.
Currently we only use the uleb128 forms. But DWARF5 also allows
1, 2, 3 and 4 byte forms (DW_FORM_strx[1234] and DW_FORM_addrx[1234])
which might be more space efficient.