backends: Handle DW_TAG_unspecified_type in dwarf_peeled_die_type

backends: Handle DW_TAG_unspecified_type in dwarf_peeled_die_type

binutils 2.40 introduces DW_TAG_unspecified_type for assembly
functions with an unknown return type. This breaks the
run-funcretval.sh testcase because dwfl_module_return_value_location
returns an error for such functions because it cannot determine the
return value location. Fix that by treating DW_TAG_unspecified_type
as if the DIE doesn't have a DW_AT_type.

Also update the testcase to explicitly checking for
DW_TAG_unspecified_type and printing "returns unspecified type".

https://sourceware.org/bugzilla/show_bug.cgi?id=30047

Signed-off-by: Mark Wielaard <mark@klomp.org>