PR28691, validate dwarf attribute form

PR28691, validate dwarf attribute form

PR28691 is a fuzzing PR that triggers a non-problem of "output changes
per run" with PIEs and/or different compilers.  I've closed similar
PRs before as wontfix, but I guess there will be no end of this type
of PR.  The trigger is an attribute that usually takes one of the
offset/constant reference DW_FORMs being given an indexed string
DW_FORM.  The bfd reader doesn't support indexed strings and returns
an error string instead.  The address of the string varies with PIE
runs and/or compiler, and we allow that address to appear in output.
Fix this by validating integer attribute forms, as we do for string
form attributes.

	PR 28691
	* dwarf2.c (is_str_attr): Rename to..
	(is_str_form): ..this.  Change param type.  Update calls.
	(is_int_form): New function.
	(read_attribute_value): Handle DW_FORM_addrx2.
	(find_abstract_instance): Validate form when using attr.u.val.
	(scan_unit_for_symbols, parse_comp_unit): Likewise.