libbpf: Make gen_loader data aligned.

Align gen_loader data to 8 byte boundary to make sure union bpf_attr,
bpf_insns and other structs are aligned.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210927145941.1383001-9-memxor@gmail.com