[libc] Search empty bits after failed allocation (#149910)
Summary:
The scheme we use to find a free bit is to just do a random walk. This
works very well up until you start to completely saturate the bitfield.
Because the result of the fetch_or yields the previous value, we can
search this to go to any known empty bits as our next guess. This
effectively increases our liklihood of finding a match after two tries
by 32x since the distribution is random.
This *massively* improves performance when a lot of memory is allocated
without freeing, as it now doesn't takea one in a million shot to fill
that last bit. A further change could improve this further by only
*mostly* filling the slab, allowing 1% to be free at all times.
