readelf: Format first DW_OP_GNU_implicit_pointer argument as DIE offset.

readelf: Format first DW_OP_GNU_implicit_pointer argument as DIE offset.

Make the printing of DW_OP_GNU_implicit_pointer consistent with other
operations that have a DIE offset as argument.

Before:
 [  6e4a]        formal_parameter
                 abstract_origin      (ref4) [  616a]
                 location             (exprloc)
                  [   0] GNU_implicit_pointer 0x6dbe, +0

After:
 [  6e4a]        formal_parameter
                 abstract_origin      (ref4) [  616a]
                 location             (exprloc)
                  [   0] GNU_implicit_pointer [  6dbe] +0

Signed-off-by: Mark Wielaard <mjw@redhat.com>