[llvm] [PowerPC ][NFC] Add a test case for the function atomic_compare_exchange_weak (PR #141263)

[Amy Kwan via llvm-commits]

================
@@ -0,0 +1,100 @@
+; Test the generate asm of function 
----------------
amy-kwan wrote:

```suggestion
; Test the generation of asm for the function:
```
Nit on comment.

https://github.com/llvm/llvm-project/pull/141263