libbpf: Fix native endian assumption when parsing BTF

Code in btf__parse_raw() fails to detect raw BTF of non-native endianness
and assumes it must be ELF data, which then fails to parse as ELF and
yields a misleading error message:

  root:/# bpftool btf dump file /sys/kernel/btf/vmlinux
  libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux

For example, this could occur after cross-compiling a BTF-enabled kernel
for a target with non-native endianness, which is currently unsupported.

Check for correct endianness and emit a clearer error message:

  root:/# bpftool btf dump file /sys/kernel/btf/vmlinux
  libbpf: non-native BTF endianness is not supported

Fixes: 94a1fedd

 ("libbpf: Add btf__parse_raw() and generic btf__parse() APIs")
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/90f81508ecc57bc0da318e0fe0f45cfe49b17ea7.1600417359.git.Tony.Ambardar@gmail.com