x86/CPU/AMD: Add X86_FEATURE_ZEN1

Add a synthetic feature flag specifically for first generation Zen
machines. There's need to have a generic flag for all Zen generations so
make X86_FEATURE_ZEN be that flag.

Fixes: 30fa9283

 ("x86/CPU/AMD: Add ZenX generations flags")
Suggested-by: Brian Gerst <brgerst@gmail.com>
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/dc3835e3-0731-4230-bbb9-336bbe3d042b@amd.com