target/i386: Define enum X86ASIdx for x86's address spaces

target/i386: Define enum X86ASIdx for x86's address spaces

Define X86ASIdx as enum, like ARM's ARMASIdx, so that it's clear index 0
is for memory and index 1 is for SMM.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Tested-By: Kirill Martynov <stdcalllevi@yandex-team.ru>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250730095253.1833411-3-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>