Use consistent type in binutils/dwarf.c

Use consistent type in binutils/dwarf.c

If you look at the type used for implicit_const objects in binutils/dwarf.c,
you'll get sometimes bfd_signed_vma and sometimes dwarf_signed_vma.

They are the same on 64-bit hosts, but not on 32-bit hosts, and the latter
discrepancy, in particular in process_abbrev_set, is responsible for the
following error issued by objdump on some object files containing DWARF 5:

binutils/dwarf.c:1108: read LEB value is too large to store in destination
variable

binutis/
	* dwarf.c (struct abbrev_attr): Change type of implicit_const.
	(add_abbrev_attr): Likewise.
	(process_abbrev_set): Likewise.
	(display_debug_abbrev): Adjust to above change.