Make string-lessp vectorisation safer

Make string-lessp vectorisation safer

Use memcpy for loading unaligned words on platforms where this can be
done efficiently.  This guards against problems arising from future
compiler autovectorisation improvements that might cause instructions
that require aligned addresses to be emitted, and should also work
with an address sanitiser enabled.

* src/fns.c (HAVE_FAST_UNALIGNED_ACCESS): Only define when optimising.
(load_unaligned_size_t): New.
(Fstring_lessp): Use load_unaligned_size_t.
* src/lisp.h (UNALIGNED_LOAD_SIZE): Remove now unused macro.