stdbuf: build on Windows (depending on cygwin dll)

[oech3]

Build Windows native stdbuf.exe depending on libstdbuf.dll built for Cygwin. Useful for MSYS2. Closes #12328

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise 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/seq/seq-epipe is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!