Simplify x86 target architecture variant handling

Instead of having two separate list of choices for select the target
architecture variant for i386 and x86_64, with many CPU choices
duplicated (because all modern x86 CPUs can be both used as i386 or
x86_64), merge them into a single list. In the x86_64 case, all the
x86 CPUs that do not support the 64 bits instruction set are hidden.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>