selftests/bpf: Allow selftests to build with older xxd

selftests/bpf: Allow selftests to build with older xxd

Currently selftests require xxd with the "-n <name>" option
which allows the user to specify a name not derived from
the input object path.  Instead of relying on this newer
feature, older xxd can be used if we link our desired name
("test_progs_verification_cert") to the input object.

Many distros ship xxd in vim-common package and do not have
the latest xxd with -n support.

Fixes: b720903e2b14d ("selftests/bpf: Enable signature verification for some lskel tests")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20251120084754.640405-3-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>