lib/util: More portable use of builtin popcnt.

lib/util: More portable use of builtin popcnt.

- Use the GCC predefined macro __POPCNT__ to detect the availability
  of fast __builtin_popcnt function.
- Use portable preprocessor macros to detect 64-bit build.
- Only define the 32-bit parts when needed and declare the
  count_1bits_8 at file scope to silence a warning.

This time I have tested all code paths to make sure no warnigns are
generated.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Reviewed-by: Simon Horman <horms@verge.net.au>