lib: utils: bitarray: Use optimized version for 32 bit array

When bitarray is limited to a single 32 bit word then bitmask_find_gap
can be used which is much faster than bit by bit search which is
used in bitarray spanning across multiple 32 bit words. Tests shows
that allocation and freeing is 2-3 times faster.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>