feat(sum): use larger buffer in bsd_sum and sysv_sum

[xtqqczze]

Follow-up #3741

Increase stack buffer size from 4 KiB to 8 KiB.

$ hyperfine --warmup 1 "time ./target/release/sum wikidatawiki-20251220-pages-logging.xml" "time ./target/release/sum.ref wikidatawiki-20251220-pages-logging.xml"
Benchmark 1: time ./target/release/sum wikidatawiki-20251220-pages-logging.xml
  Time (mean ± σ):     12.819 s ±  0.012 s    [User: 8.082 s, System: 2.085 s]
  Range (min … max):   12.807 s … 12.845 s    10 runs
 
Benchmark 2: time ./target/release/sum.ref wikidatawiki-20251220-pages-logging.xml
  Time (mean ± σ):     13.085 s ±  0.022 s    [User: 8.138 s, System: 2.290 s]
  Range (min … max):   13.065 s … 13.142 s    10 runs
 
Summary
  time ./target/release/sum wikidatawiki-20251220-pages-logging.xml ran
    1.02 ± 0.00 times faster than time ./target/release/sum.ref wikidatawiki-20251220-pages-logging.xml

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[cakebaker]

Thanks!