util: Make popcount() handle 64-bit integers, not separate popcount64().

util: Make popcount() handle 64-bit integers, not separate popcount64().

Having a single function that can do popcount() on any integer type is
easier for callers to get right.  The implementation is probably slower
if the caller actually provides a 32-bit (or shorter) integer, but the
only existing callers always provide a full 64-bit integer so this seems
unimportant for now.

This also restores use, in practice, of the optimized implementation of
population count.  (As the comment on popcount32() says, this version is
2x faster than __builtin_popcount().)

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>