cmake: support multiple CPU targets

cmake/checks/cpu_avx2.cpp

@@ -0,0 +1,10 @@
+#if !defined __AVX2__ // MSVC supports this flag since MSVS 2013
+#error "__AVX2__ define is missing"
+#endif
+#include <immintrin.h>
+void test()
+{
+    int data[8] = {0,0,0,0, 0,0,0,0};
+    __m256i a = _mm256_loadu_si256((const __m256i *)data);
+}
+int main() { return 0; }