target/i386: Mask CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs

target/i386: Mask CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs

Zhaoxin CPUs (including vendors "Shanghai" and "Centaurhauls") handle the
CMPLegacy bit similarly to Intel CPUs. Therefore, this commit masks the
CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs, just as it is done
for Intel CPUs.

AMD uses the CMPLegacy bit (CPUID[0x80000001].ECX.bit1) along with other CPUID
information to enumerate platform topology (e.g., the number of logical
processors per package). However, for Intel and other CPUs that follow Intel's
behavior, CPUID[0x80000001].ECX.bit1 is reserved.

- Impact on Intel and similar CPUs:
This change has no effect on Intel and similar CPUs, as the goal is to
accurately emulate CPU CPUID information.

- Impact on Linux Guests running on Intel (and similar) vCPUs:
During boot, Linux checks if the CPU supports Hyper-Threading. For the Linux
kernel before v6.9, if it detects X86_FEATURE_CMP_LEGACY, it assumes
Hyper-Threading is not supported. For Intel and similar vCPUs, if the
CMPLegacy bit is not masked in CPUID[0x80000001].ECX, Linux will incorrectly
assume that Hyper-Threading is not supported, even if the vCPU does support it.

Signed-off-by: EwanHai <ewanhai-oc@zhaoxin.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250113074413.297793-5-ewanhai-oc@zhaoxin.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>