selftests/bpf: test_xdp_meta: Rename BPF sections

SEC("t") and SEC("x") can't be loaded by the __load() helper.

Rename these sections SEC("tc") and SEC("xdp") so they can be
interpreted by the __load() helper in upcoming patch.
Update the test_xdp_meta.sh to fit these new names.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20241213-xdp_meta-v2-1-634582725b90@bootlin.com