bpf: Use option "help" in the llvm-objcopy test

I noticed the "--version" option of the llvm-objcopy command has recently
disappeared from the master llvm branch.  It is currently used as a BTF
support test in tools/testing/selftests/bpf/Makefile.

This patch replaces it with "--help" which should be
less error prone in the future.

Fixes: c0fa1b6c ("bpf: btf: Add BTF tests")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>