dwarf.c (process_abbrev_set): Properly parenthesize, fix fallout

Building as ILP32 shows:

 gcc -m32 -DHAVE_CONFIG_H -I. -I/checkout/binutils  -I. -I/checkout/binutils -I../bfd -I/checkout/binutils/../bfd -I/checkout/binutils/../include -DLOCALEDIR="\"/usr/local/share/locale\"" -Dbin_dummy_emulation=bin_vanilla_emulation  -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow -Wstack-usage=262144 -Werror -I/checkout/binutils/../zlib -g -O2   -MT dwarf.o -MD -MP -MF $depbase.Tpo -c -o dwarf.o /checkout/binutils/dwarf.c &&\
 mv -f $depbase.Tpo $depbase.Po
 In file included from /checkout/binutils/sysdep.h:101:0,
                  from /checkout/binutils/dwarf.c:21:
 /checkout/binutils/dwarf.c: In function 'process_abbrev_set':
 /checkout/binutils/dwarf.c:1072:15: error: format '%lx' expects argument of type 'long unsigned int', but argument 2 has type 'dwarf_vma {aka long long unsigned int}' [-Werror=format=]
        warn (_("Debug info is corrupted, abbrev size (%lx) is larger than "
                ^
 /checkout/binutils/dwarf.c:1072:13: note: in expansion of macro '_'
        warn (_("Debug info is corrupted, abbrev size (%lx) is larger than "
              ^
 cc1: all warnings being treated as errors
 Makefile:1101: recipe for target 'dwarf.o' failed

The recent commit, casting one of the terms, has an obvious
typo.  To wit, the (non-cast) term abbrev_size is a
dwarf_vma and causes the whole expression to (still) be 64
bits.

binutils:
	* dwarf.c (process_abbrev_set): Properly parenthesize before
	casting to unsigned long.