cutils: add popcount/popcountl/popcountll static inline definitions

Just pass them through to their specific GCC builtins. If we find
that the builtins are not performing well or have other issues, we can
pull in existing public implementations and/or write arch-specific
helpers if necessary.

Change-Id: I278e6b73d75fac88808ed4546070d288c06f79e0
Signed-off-by: Dima Zavin <dima@android.com>