x86 bpf_jit: support BPF_S_ANC_ALU_XOR_X instruction

commit ffe06c17  (filter: add XOR operation) added generic support
for XOR operation.

This patch implements the XOR instruction in x86 jit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>