lib_generic memset: fill one word at a time if possible

If the destination is aligned, fill ulong values until possible.
Then fill remaining part by byte.

Signed-off-by: Alessandro Rubini <rubini@unipv.it>
Acked-by: Andrea Gallo <andrea.gallo@stericsson.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>