eBPF llvm backend can support different BPF formats, make sure the object
we're trying to load matches with regards to endiannes and while at it, also
check for other attributes related to BPF ELFs.

  # llc --version
  LLVM (http://llvm.org/):
    LLVM version 3.8.0svn
    Optimized build.
    Built Jan  9 2016 (02:08:10).
    Default target: x86_64-unknown-linux-gnu
    Host CPU: ivybridge

    Registered Targets:
      bpf    - BPF (host endian)
      bpfeb  - BPF (big endian)
      bpfel  - BPF (little endian)
      [...]
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>