libbpf: Reject static entry-point BPF programs

Detect use of static entry-point BPF programs (those with SEC() markings) and
emit error message. This is similar to
c1cccec9 ("libbpf: Reject static maps") but for BPF programs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210514195534.1440970-1-andrii@kernel.org