selftests/bpf: mark dynptr kfuncs __weak to make them optional on old kernels

selftests/bpf: mark dynptr kfuncs __weak to make them optional on old kernels

Mark dynptr kfuncs as __weak to allow
verifier_global_subprogs/arg_ctx_{perf,kprobe,raw_tp} subtests to be
loadable on old kernels. Because bpf_dynptr_from_xdp() kfunc is used
from arg_tag_dynptr BPF program in progs/verifier_global_subprogs.c
*and* is not marked as __weak, loading any subtest from
verifier_global_subprogs fails on old kernels that don't have
bpf_dynptr_from_xdp() kfunc defined. Even if arg_tag_dynptr program
itself is not loaded, libbpf bails out on non-weak reference to
bpf_dynptr_from_xdp (that can't be resolved), which shared across all
programs in progs/verifier_global_subprogs.c.

So mark all dynptr-related kfuncs as __weak to unblock libbpf CI ([0]).
In the upcoming "kfunc in vmlinux.h" work we should make sure that
kfuncs are always declared __weak as well.

  [0] https://github.com/libbpf/libbpf/actions/runs/7792673215/job/21251250831?pr=776#step:4:7961

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240206004008.1541513-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>