libbpf: Add "tc" SEC_DEF which is a better name for "classifier"

As argued in [0], add "tc" ELF section definition for SCHED_CLS BPF
program type. "classifier" is a misleading terminology and should be
migrated away from.

  [0] https://lore.kernel.org/bpf/270e27b1-e5be-5b1c-b343-51bd644d0747@iogearbox.net/Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210928161946.2512801-2-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -7968,6 +7968,7 @@ static const struct bpf_sec_def section_defs[] = {
 		.attach_fn = attach_kprobe),
 	BPF_PROG_SEC("uretprobe/",		BPF_PROG_TYPE_KPROBE),
 	BPF_PROG_SEC("classifier",		BPF_PROG_TYPE_SCHED_CLS),
+	BPF_PROG_SEC("tc",			BPF_PROG_TYPE_SCHED_CLS),
 	BPF_PROG_SEC("action",			BPF_PROG_TYPE_SCHED_ACT),
 	SEC_DEF("tracepoint/", TRACEPOINT,
 		.attach_fn = attach_tp),