[llvm] [AIX][AsmPrinter] Fix unsigned subtraction wrap-around (PR #122214)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-cmake-x86_64-avx512-linux` running on `avx512-intel64` while building `llvm` at step 1 "Checkout lnt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/133/builds/9352

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

```
Step 1 (Checkout lnt) failure: update (failure)
git version 2.31.1
>From https://github.com/llvm/llvm-lnt
 * branch            HEAD       -> FETCH_HEAD
fatal: unable to write new index file
>From https://github.com/llvm/llvm-lnt
 * branch            HEAD       -> FETCH_HEAD
fatal: unable to write new index file

```

</details>

https://github.com/llvm/llvm-project/pull/122214