target-i386: fix decoding of negative 4-byte displacements

Negative four byte displacements need to be sign-extended after
c086b783eb7a578993d6d2ab62c4c2666800b63d.  Do so.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Richard Henderson  <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
