[libc][malloc] Reduce block overhead by 4 bytes plus alignment effects (#99945)

The unused padding and alignment fields were removed. The used and last
bits were stashed into the lower two bits of the next chunk offset.
(This is a very typical trick for Knuth boundary tags.) The chunk
offsets were recast as counting bytes rather than multiples of the
alignment. To ensure that the lowest two bits are not significant, the
minimum alignment was bumped to 4. This shouldn't affect anything in
practice, since alignof(max_align_t) is overwhelmingly likely to be 8.

See #98096

0 files changed, 0 insertions, 0 deletions