If the compiler knows of a better algorithm for counting the number of
bits set in a word for the target CPU, let it use that, instead of the
classic algorithm optimized for PDP-6.

Based on libXext commit 490a25e6f8a4d2482af4364c700b68ad11a4d10b

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>