[bpf] add bigendian support to disassembler

. swap 4-bit register encoding, 16-bit offset and 32-bit imm to support big endian archs
. add a test

Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 301653