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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-debian-fast` running on `gribozavr4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/26948

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
...
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-reloc-symb.mir (40085 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix32-vector-vararg-fixed-callee.ll (40086 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-vec_extract_p9_2.ll (40087 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-lower-block-address.ll (40088 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-lower-constant-pool-index.ll (40089 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix64-csr-alloc.mir (40090 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix32-vector-pair-cc-spills.ll (40091 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-endian-error.ll (40092 of 83743)
PASS: LLVM :: CodeGen/PowerPC/allocate-r0.ll (40093 of 83743)
UNRESOLVED: LLVM :: CodeGen/PowerPC/atomic-compare-exchange-weak.ll (40094 of 83743)
******************** TEST 'LLVM :: CodeGen/PowerPC/atomic-compare-exchange-weak.ll' FAILED ********************
Test has unterminated 'RUN:' directive (with '\') from line 7 to 9
********************
PASS: LLVM :: CodeGen/PowerPC/alloca-oversized.ll (40095 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-tls-gd-int.ll (40096 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-data-only-notoc.ll (40097 of 83743)
PASS: LLVM :: CodeGen/PowerPC/align.ll (40098 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-roptr.ll (40099 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-explicit-section.ll (40100 of 83743)
PASS: LLVM :: CodeGen/PowerPC/and-branch.ll (40101 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-tls-le-ldst-O0.ll (40102 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-symbol-rename.ll (40103 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-reloc-large.ll (40104 of 83743)
PASS: LLVM :: CodeGen/PowerPC/altivec-ord.ll (40105 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix64-virtual-call-no-spills.ll (40106 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-cc-abi.ll (40107 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-used-with-stringpool.ll (40108 of 83743)
PASS: LLVM :: CodeGen/PowerPC/and-extend-combine.ll (40109 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-tls-gd-double.ll (40110 of 83743)
PASS: LLVM :: CodeGen/PowerPC/anyext_srl.ll (40111 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix64-vector-pair-cc-spills.ll (40112 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-tls-gd-longlong.ll (40113 of 83743)
PASS: LLVM :: CodeGen/PowerPC/alignlongjumptest.mir (40114 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix32-cc-abi-vaarg.ll (40115 of 83743)
PASS: LLVM :: CodeGen/PowerPC/anon_aggr.ll (40116 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-exception-section.ll (40117 of 83743)
PASS: LLVM :: CodeGen/PowerPC/and_sra.ll (40118 of 83743)
PASS: LLVM :: CodeGen/PowerPC/and-imm.ll (40119 of 83743)
PASS: LLVM :: CodeGen/PowerPC/arr-fp-arg-no-copy.ll (40120 of 83743)
PASS: LLVM :: CodeGen/PowerPC/arg_promotion.ll (40121 of 83743)
PASS: LLVM :: CodeGen/PowerPC/ashr-neg1.ll (40122 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-mergeable-const.ll (40123 of 83743)
PASS: LLVM :: CodeGen/PowerPC/asm-printer-topological-order.ll (40124 of 83743)
PASS: LLVM :: CodeGen/PowerPC/aix-xcoff-lower-comm.ll (40125 of 83743)
PASS: LLVM :: CodeGen/PowerPC/alias.ll (40126 of 83743)
PASS: LLVM :: CodeGen/PowerPC/and-elim.ll (40127 of 83743)
PASS: LLVM :: CodeGen/PowerPC/asm-Zy.ll (40128 of 83743)
PASS: LLVM :: CodeGen/PowerPC/andc.ll (40129 of 83743)
PASS: LLVM :: CodeGen/PowerPC/asm-template-I.ll (40130 of 83743)

```

</details>

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