libbpf: Enforce 64-bitness of BTF for BPF object files

BPF object files are always targeting 64-bit BPF target architecture, so
enforce that at BTF level as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200813204945.1020225-7-andriin@fb.com