tools: bpf_jit_disasm: Add option to dump JIT image to a file.

When debugging the JIT on an embedded platform or cross build
environment, libbfd may not be available, making it impossible to run
bpf_jit_disasm natively.

Add an option to emit a binary image of the JIT code to a file.  This
file can then be disassembled off line.  Typical usage in this case
might be (pasting mips64 dmesg output to cat command):

   $ cat > jit.raw
   $ bpf_jit_disasm -f jit.raw -O jit.bin
   $ mips64-linux-gnu-objdump -D -b binary -m mips:isa64r2 -EB jit.bin

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>