[AIX] Remove unsupported AIX native echo option -n (#199079)
AIX native echo doesn't support the `-n` flag.
Use the POSIX-standard `\c` escape sequence instead to suppress the
trailing newline, ensuring the test works across all systems and make it
portable.
The current test fails as follows:
```
FAIL: lit :: unit/Util.py (1 of 1)
******************** TEST 'lit :: unit/Util.py' FAILED ********************
Exit Code: 1
Command Output (stdout):
--
# RUN: at line 1
"/opt/freeware/bin/python3.12" /home/himadhit/llvm/community/build/utils/lit/tests/unit/Util.py
# executed command: /opt/freeware/bin/python3.12 /home/himadhit/llvm/community/build/utils/lit/tests/unit/Util.py
# .---command stderr------------
# | F..
# | ======================================================================
# | FAIL: test_basic (__main__.TestCommandCache.test_basic)
# | ----------------------------------------------------------------------
# | Traceback (most recent call last):
# | File "/home/himadhit/llvm/community/build/utils/lit/tests/unit/Util.py", line 32, in test_basic
# | self.assertEqual(lit_config.run_command_cached(["echo", "hi"]), b"hi")
# | AssertionError: b'hi\n' != b'hi'
# |
# | ----------------------------------------------------------------------
# | Ran 3 tests in 2.050s
# |
# | FAILED (failures=1)
# `-----------------------------
# error: command failed with exit status: 1
```
---------
Co-authored-by: himadhith <himadhith.v@ibm.com>