cp: improve throughput with pipe input (Linux only)

[oech3]

$ truncate -s 9PB /tmp/huge
$ cat /tmp/huge | target/debug/cp /dev/stdin /dev/stdout | pv>/dev/null
[35.7GiB/s]
$ cat /tmp/huge | cp /dev/stdin /dev/stdout | pv>/dev/null
[9.83GiB/s]

[github-actions]

GNU testsuite comparison:

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