The fix updates the llvm-strings stdin lit tests to use `printf` instead
of `echo -n` when generating input via stdin.
The behavior of `echo -n` is not portable and varies across shells and
platforms, particularly on AIX where `/bin/sh` handles `-n` and escape
processing differently from GNU-compatible shells. This causes the tests
to fail despite correct llvm-strings functionality.
In contrast, `printf` has well-defined, POSIX-specified semantics and
provides consistent handling of newlines, escape sequences, and
non-printable characters. Using `printf` eliminates the dependency on
shell-specific `echo` behavior and makes the tests deterministic.
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