[llvm] Revert changes to llvm-string stdin test and XFAIL on AIX (PR #160821)

[James Henderson via llvm-commits]

================
@@ -1,13 +1,13 @@
+; XFAIL: target={{.*}}-aix{{.*}}
----------------
jh7370 wrote:

This isn't correct. It'll XFAIL the test if the AIX target is enabled, when it should be when the host system is AIX.

https://github.com/llvm/llvm-project/pull/160821