Workaround using unaligned loads for gcc < 9

despite usage of alignas, the generated (avx2/avx512) code with older compilers needs to use
unaligned loads with older gcc (e.g. confirmed crash with gcc 7.3/mingw on abrok).

Better performance thus requires gcc >= 9 on hardware supporting avx2/avx512

closes https://github.com/official-stockfish/Stockfish/pull/2969

No functional change

src/nnue/nnue_common.h

@@ -37,6 +37,27 @@
 #include <arm_neon.h>
 #endif
 
+// HACK: Use _mm256_loadu_si256() instead of _mm256_load_si256. Otherwise a binary
+//       compiled with older g++ crashes because the output memory is not aligned
+//       even though alignas is specified.
+#if defined(USE_AVX2)
+#if defined(__GNUC__ ) && (__GNUC__ < 9)
+#define _mm256_loadA_si256  _mm256_loadu_si256
+#define _mm256_storeA_si256 _mm256_storeu_si256
+#else
+#define _mm256_loadA_si256  _mm256_load_si256
+#define _mm256_storeA_si256 _mm256_store_si256
+#endif
+#endif
+
+#if defined(USE_AVX512)
+#if defined(__GNUC__ ) && (__GNUC__ < 9)
+#define _mm512_loadA_si512  _mm512_loadu_si512
+#else
+#define _mm512_loadA_si512  _mm512_load_si512
+#endif
+#endif
+
 namespace Eval::NNUE {
 
   // Version of the evaluation file