x86/CPU/AMD: Add X86_FEATURE_ZEN6

Add a synthetic feature flag for Zen6.

  [  bp: Move the feature flag to a free slot and avoid future merge
     conflicts from incoming stuff. ]

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250513204857.3376577-1-yazen.ghannam@amd.com