[binutils] Handle DW_FORM_ref_sig8 in get_type_abbrev_from_form

[binutils] Handle DW_FORM_ref_sig8 in get_type_abbrev_from_form

When compiling an exec like this:
...
$ gcc -fdebug-types-section hello.c -gdwarf-5
...
we run into:
...
$ readelf -w a.out > READELF
readelf: Warning: Unexpected form 20 encountered whilst finding \
  abbreviation for type
...

Fix this by handling DW_FORM_ref_sig8 conservatively in
get_type_abbrev_from_form.

binutils/ChangeLog:

2021-02-09  Tom de Vries  <tdevries@suse.de>

	PR binutils/27370
	* dwarf.c (get_type_abbrev_from_form): Handle DW_FORM_ref_sig8.