x86-64: Fix INSN_JMP_MEMINDEX encoding

Table 2-5 ("Special Cases of REX Encodings") of Intel manual states
that:

  EBP without a displacement must be done using mod = 01 with
  displacement of 0.

The problem is visible on x86-64 where INSN_JMP_MEMINDEX is encoded
wrong. This fixes issue #34.

Signed-off-by: Pekka Enberg <penberg@kernel.org>