i386/cpu: Fix CPUID[0x80000006] for Intel CPU

i386/cpu: Fix CPUID[0x80000006] for Intel CPU

Per SDM, Intel supports CPUID[0x80000006]. But only L2 information is
encoded in ECX (note that L2 associativity field encodings rules
consistent with AMD are used), all other fields are reserved.

Therefore, make the following changes to CPUID[0x80000006]:
 * Check the vendor in CPUID[0x80000006] and just encode L2 to ECX for
   Intel.
 * Drop the lines_per_tag assertion, since AMD supports this field but
   Intel doesn't. And this field can be easily checked via cpuid tool
   in Guest.
 * Apply the encoding change of Intel for Zhaoxin as well [1].

This fix also resolves the FIXME of legacy_l2_cache_amd:

/*FIXME: CPUID leaf 0x80000006 is inconsistent with leaves 2 & 4 */

In addition, per AMD's APM, update the comment of CPUID[0x80000006].

[1]: https://lore.kernel.org/qemu-devel/c522ebb5-04d5-49c6-9ad8-d755b8998988@zhaoxin.com/

Tested-by: Yi Lai <yi1.lai@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250711102143.1622339-11-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>