maint: avoid -fsanitize=undefined warning in rand-isaac

* gl/lib/rand-isaac.c (isaac_refill): readisaac() purposefully passes
unaligned pointers to avoid memory copies.  This is only done on
platforms where this is defined, so avoid the associated
runtime warning generated with -fsanitize=undefined, which is:

  lib/rand-isaac.c:125:182: runtime error: store to misaligned address
  0x63100003d7fd for type 'isaac_word', which requires 8 byte alignment
  0x63100003d7fd: note: pointer points here
   47 ce ed a4 be be be  00 00 00 00 00 00 00 00  ...
               ^