Minor tweak to use die_reader_specs::abfd

Minor tweak to use die_reader_specs::abfd

For an experiment I'm working on, it would be convenient if
die_reader_specs::cu could be NULL.  This is fairly involved to
implement, but I did notice one spot that could conveniently be
updated.  While making this trivial change, I also noticed a small,
related formatting error.

2021-03-14  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (struct die_reader_specs) <abfd>: Fix formatting.
	(peek_die_abbrev): Use reader.abfd.