libbpf: clean up SEC() handling

Get rid of sloppy prefix logic and remove deprecated xdp_{devmap,cpumap}
sections.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220627211527.2245459-13-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>