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 xorg/lib/libxext@490a25e6f8a4d2482af4364c700b68ad11a4d10b
v2: make old version static inline, stop exporting after !1695
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1674>