x86/cpu/vfm: Add/initialize x86_vfm field to struct cpuinfo_x86

Refactor struct cpuinfo_x86 so that the vendor, family, and model
fields are overlaid in a union with a 32-bit field that combines
all three (together with a one byte reserved field in the upper
byte).

This will make it easy, cheap, and reliable to check all three
values at once.

See

  https://lore.kernel.org/r/Zgr6kT8oULbnmEXx@agluck-desk3

for why the ordering is (low-to-high bits):

  (vendor, family, model)

  [ bp: Move comments over the line, add the backstory about the
    particular order of the fields. ]
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240416211941.9369-2-tony.luck@intel.com

arch/x86/include/asm/processor.h

@@ -108,9 +108,23 @@ struct cpuinfo_topology {
 };
 
 struct cpuinfo_x86 {
-	__u8			x86;		/* CPU family */
-	__u8			x86_vendor;	/* CPU vendor */
-	__u8			x86_model;
+	union {
+		/*
+		 * The particular ordering (low-to-high) of (vendor,
+		 * family, model) is done in case range of models, like
+		 * it is usually done on AMD, need to be compared.
+		 */
+		struct {
+			__u8	x86_model;
+			/* CPU family */
+			__u8	x86;
+			/* CPU vendor */
+			__u8	x86_vendor;
+			__u8	x86_reserved;
+		};
+		/* combined vendor, family, model */
+		__u32		x86_vfm;
+	};
 	__u8			x86_stepping;
 #ifdef CONFIG_X86_64
 	/* Number of 4K pages in DTLB/ITLB combined(in pages): */