AVX 512 detection: workaround for older GCC

cmake/checks/cpu_avx512skx.cpp

 #if defined __AVX512__ || defined __AVX512F__
 #include <immintrin.h>
+
+// Workaround for problem with GCC 5-6 in -O0 mode
+struct v_uint32x16
+{
+    __m512i val;
+    explicit v_uint32x16(__m512i v) : val(v) {}
+};
+inline v_uint32x16 operator << (const v_uint32x16& a, int imm)
+{
+    return v_uint32x16(_mm512_slli_epi32(a.val, imm));
+}
+
 void test()
 {
     __m512i zmm = _mm512_setzero_si512();
@@ -7,10 +19,13 @@ void test()
     __m256i b = _mm256_abs_epi64(a); // VL
     __m512i c = _mm512_abs_epi8(zmm); // BW
     __m512i d = _mm512_broadcast_i32x8(b); // DQ
+    v_uint32x16 e(d); e = e << 10;
+    __m512i f = _mm512_packus_epi32(d,d);
 #if defined __GNUC__ && defined __x86_64__
     asm volatile ("" : : : "zmm16", "zmm17", "zmm18", "zmm19");
 #endif
 }
+
 #else
 #error "AVX512-SKX is not supported"
 #endif