Workaround for missing intrinsic on gcc < 9 (facebookresearch#3481)

Summary:
Rebased branch for facebookresearch#3420

Pull Request resolved: facebookresearch#3481

Reviewed By: mdouze

Differential Revision: D57830230

Pulled By: junjieqi

fbshipit-source-id: a93fb3cc53f11245faec891a9590b5e849dbf3b9

Author: borrrden

faiss/impl/code_distance/code_distance-avx2.h

@@ -16,6 +16,11 @@
 #include <faiss/impl/ProductQuantizer.h>
 #include <faiss/impl/code_distance/code_distance-generic.h>
 
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78782
+#if defined(__GNUC__) && __GNUC__ < 9
+#define _mm_loadu_si64(x) (_mm_loadl_epi64((__m128i_u*)x))
+#endif
+
 namespace {
 
 inline float horizontal_sum(const __m128 v) {