runtime (gc_blocks.go): clear full size of allocation

[niaow]

This fixes a bug where runtime.alloc would not clear the padding from rounding the allocation up to a multiple of the block size. The GC still has to scan this, and this could result in permanent false positives (and therefore memory leaks).

It also handles overflow in the size calculation.

[niaow]

Performance impact on the go/format benchmark that was struggling earlier:

                    │  precise.txt  │          precise-fixed.txt           │              boehm.txt              │
                    │    sec/op     │    sec/op     vs base                │   sec/op     vs base                │
Format/array1-10000   44.57m ± 158%   27.77m ± 20%  -37.68% (p=0.000 n=10)   21.01m ± 8%  -52.85% (p=0.000 n=10)

                    │  precise.txt  │           precise-fixed.txt           │               boehm.txt               │
                    │      B/s      │      B/s       vs base                │     B/s       vs base                 │
Format/array1-10000   1.392Mi ± 62%   2.227Mi ± 17%  +59.93% (p=0.000 n=10)   2.942Mi ± 9%  +111.30% (p=0.000 n=10)

[deadprogram]

That is one very subtle and difficult bug @niaow great working finding and fixing it! Now merging.