Apparently, in stats_reset_table(), the innocuous

  memset(&group->counters, 0, sizeof(group->counters));

is converted by clang to SSE2 instructions.

The problem is that "group" is not correctly aligned,
despite  MY_ALIGNED(CPU_LEVEL1_DCACHE_LINESIZE) in the thread_group_t
declaration.

It is not aligned because it was allocated with my_malloc, since
commit fd9f1638, MDEV-5205. Previously all_groups was a
statically allocated array.

Fix is to remove MY_ALIGNED, and pad the struct instead.