Read AVX2 from the processor

Avx2.IsSupported says whether the starter's own code may use AVX2, and the AOT
compiler fixes that at build time to its baseline, which has none. A published
starter told every player that their processor lacked AVX2. The check now reads
CPUID: AVX2 in leaf 7, AVX and OSXSAVE in leaf 1.

StarterNG/Infrastructure/CpuFeatures.cs

@@ -12,5 +12,21 @@ public static class CpuFeatures
     /// processor. Other architectures cannot run them at all, which is not this.
     /// </summary>
     public static bool LacksAvx2 =>
-        RuntimeInformation.ProcessArchitecture is Architecture.X64 or Architecture.X86 && !Avx2.IsSupported;
+        RuntimeInformation.ProcessArchitecture is Architecture.X64 or Architecture.X86 && !HasAvx2();
+
+    /// <summary>
+    /// Asks the processor itself. Avx2.IsSupported only says whether the starter's own
+    /// code may use AVX2, and the AOT compiler fixes that at build time to its baseline,
+    /// which has no AVX2, so a published starter always read false.
+    /// </summary>
+    static bool HasAvx2()
+    {
+        if (!X86Base.IsSupported || X86Base.CpuId(0, 0).Eax < 7)
+            return false;
+        // Leaf 1 ECX: bit 27 the system saves the extended registers (OSXSAVE), bit 28 AVX.
+        // Leaf 7 EBX: bit 5 AVX2.
+        const int osxsaveAndAvx = (1 << 27) | (1 << 28);
+        return (X86Base.CpuId(1, 0).Ecx & osxsaveAndAvx) == osxsaveAndAvx &&
+               (X86Base.CpuId(7, 0).Ebx & (1 << 5)) != 0;
+    }
 }