Restrict tests in Transforms/InstCombine/SystemZ to SystemZ (NFC) (llvm#146363)

This commit adds a `lit.local.cfg` file to
`llvm/test/Transforms/InstCombine/SystemZ` that makes sure the tests
contained in that folder are only run when `SystemZ` is among the
supported targets.

Author: dominik-steenken

llvm/test/Transforms/InstCombine/SystemZ/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "SystemZ" in config.root.targets:
+    config.unsupported = True