x86: LEA instruction

Signed-off-by: Pekka Enberg <penberg@iki.fi>
x86: Fix relative address decoding

Commit 13da3c7 ("x86: Conditional jump instructions") broke decoding of
negative relative addresses.

Fix that up.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
x86: Simplify decode_disp()

Signed-off-by: Pekka Enberg <penberg@iki.fi>