od: improve perf for --skip-bytes non-seekable input by splice(2)

[oech3]

Closes #12598
This PR adds splice fast-path for --skip-bytes for non-seekable input as same as wc -c.

> truncate -s 1GB /tmp/huge
> time cat /tmp/huge|target/release/od-splice --skip-bytes 1GB
7346545000

________________________________________________________
Executed in   17.97 millis    fish           external
   usr time    0.95 millis    0.00 millis    0.95 millis
   sys time   31.25 millis    3.02 millis   28.23 millis

> time cat /tmp/huge|target/release/od --skip-bytes 1GB
7346545000

________________________________________________________
Executed in  160.61 millis    fish           external
   usr time  109.92 millis    0.89 millis  109.03 millis
   sys time  181.50 millis    2.76 millis  178.73 millis

> hyperfine "cat /tmp/huge|target/release/od --skip-bytes 1GB" "cat /tmp/huge|target/release/od-splice --skip-bytes 1GB" 
Benchmark 1: cat /tmp/huge|target/release/od --skip-bytes 1GB
  Time (mean ± σ):     155.4 ms ±  26.3 ms    [User: 105.1 ms, System: 183.9 ms]
  Range (min … max):   141.6 ms … 260.3 ms    20 runs
 
Benchmark 2: cat /tmp/huge|target/release/od-splice --skip-bytes 1GB
  Time (mean ± σ):      18.9 ms ±   6.1 ms    [User: 3.6 ms, System: 27.3 ms]
  Range (min … max):    15.5 ms …  68.1 ms    154 runs
 
Summary
  cat /tmp/huge|target/release/od-splice --skip-bytes 1GB ran
    8.24 ± 3.00 times faster than cat /tmp/huge|target/release/od --skip-bytes 1GB

[codspeed-hq]

Merging this PR will not alter performance

✅ 347 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing oech3:splice-discard (b170008) with main (ab34a45)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/parent-perm-race. tests/cp/parent-perm-race is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/cp/sparse-to-pipe. tests/cp/sparse-to-pipe is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 148.56 MB (+968 KB, +0.64%)

Significant per-binary changes:
  ls         2.11 MB ->    3.25 MB  (+1.14 MB, +54.36%)
  mkfifo     1.16 MB ->    1.08 MB  (-76 KB, -6.42%)
  mknod      1.16 MB ->    1.09 MB  (-76 KB, -6.38%)
  mkdir      1.16 MB ->    1.09 MB  (-72 KB, -6.04%)