PS: It's STRD, which as far as I understand the Arm Architecture Reference Manual always requires word alignment.

  Program received signal SIGBUS, Bus error.
  small_memset (n=<optimized out>, c=<optimized out>, s=0x29690)
      at /home/pi/memset_benchmark/src/lib.c:33
  33          *((uint64_t *)last) = val8;
  1: x/i $pc
  => 0x11c8c <local_memset+1560>: strd    r0, [r7, #-8]
  (gdb) info registers
  r0             0x0                 0
  r1             0x0                 0
  r2             0x0                 0
  r3             0x1475              5237
  r4             0x5f5e100           100000000
  r5             0x11674             71284
  r6             0x29690             169616
  r7             0x29699             169625