bitops: speed up hweight<N>()

bitops: speed up hweight<N>()

Algorithmically this gets us in line with current Linux, where the same
change did happen about 13 years ago. See in particular Linux commits
f9b4192923 ("bitops: hweight() speedup") and 0136611c62 ("optimize
hweight64 for x86_64").

Kconfig changes for actually setting HAVE_FAST_MULTIPLY will follow.

Take the opportunity and change generic_hweight64()'s return type to
unsigned int.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>