MDEV-19709 Bitmap<128>::merge etc may crash on older GCC versions

Older GCC generates SSE instruction on not-128-bit-aligned data in
Bitmap<128>::buffer

Workaround by forcing GCC not to use SSE on Bitmap<N> template.

sql/sql_bitmap.h

@@ -27,6 +27,18 @@
 #include <my_bitmap.h>
 #include <my_bit.h>
 
+/*
+  Workaround GCC optimizer bug (generating SSE instuctions on unaligned data)
+*/
+#if defined (__GNUC__) && defined(__x86_64__) && (__GNUC__ < 6)
+#define NEED_GCC_NO_SSE_WORKAROUND
+#endif
+
+#ifdef NEED_GCC_NO_SSE_WORKAROUND
+#pragma GCC push_options
+#pragma GCC target ("no-sse")
+#endif
+
 template <uint width> class Bitmap
 {
   uint32 buffer[(width + 31) / 32];
@@ -266,6 +278,11 @@ template <uint width> class Bitmap
   };
 };
 
+#ifdef NEED_GCC_NO_SSE_WORKAROUND
+#pragma GCC pop_options
+#undef NEED_GCC_NO_SSE_WORKAROUND
+#endif
+
 /* An iterator to quickly walk over bits in ulonglong bitmap. */
 class Table_map_iterator
 {