Correctly check for AVX support in x64CPUDetect

It's not enough to check for the CPUID bit to know if AVX is supported since
AVX requires OS support (new set of registers == more registers to be saved
when context switching). If the OS does not support, the cpuid bit will still
be set but using YMM registers will cause an illegal exception fault.

Source/Core/Common/Src/x64CPUDetect.cpp

@@ -72,6 +72,14 @@ static void __cpuid(int info[4], int x)
 #endif
 }
 
+#define _XCR_XFEATURE_ENABLED_MASK 0
+static unsigned long long _xgetbv(unsigned int index)
+{
+	unsigned int eax, edx;
+	__asm__ __volatile__("xgetbv" : "=a"(eax), "=d"(edx) : "c"(index));
+	return ((unsigned long long)edx << 32) | eax;
+}
+
 #endif
 
 #include "Common.h"
@@ -149,8 +157,17 @@ void CPUInfo::Detect()
 		if ((cpu_id[2] >> 9)  & 1) bSSSE3 = true;
 		if ((cpu_id[2] >> 19) & 1) bSSE4_1 = true;
 		if ((cpu_id[2] >> 20) & 1) bSSE4_2 = true;
-		if ((cpu_id[2] >> 28) & 1) bAVX = true;
 		if ((cpu_id[2] >> 25) & 1) bAES = true;
+
+		// AVX support requires 3 separate checks:
+		//  - Is the AVX bit set in CPUID?
+		//  - Is the XSAVE bit set in CPUID?
+		//  - XGETBV result has the XCR bit set.
+		if (((cpu_id[2] >> 28) & 1) && ((cpu_id[2] >> 27) & 1))
+		{
+			if (_xgetbv(_XCR_XFEATURE_ENABLED_MASK) & 0x6)
+				bAVX = true;
+		}
 	}
 	if (max_ex_fn >= 0x80000004) {
 		// Extract brand string