target-i386: fix SIB decoding with index = 4

target-i386: fix SIB decoding with index = 4

A SIB byte with an index of 4 means "no scaled index", even if the scale
value is not 0. In 64-bit mode, if REX.X is used, an index of 4 selects
%r12. This is correctly handled by the computation of the index variable,
which includes the index bits, and also the REX.X prefix:

    index = ((code >> 3) & 7) | REX_X(s);

Thanks to Avi Kivity, Jamie Lokier and Malc for the analysis of the
problem and the initial patch.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit b16f827bdf7444b8cd338b9ecb654b4752f47225)