[llvm] [AIX][CMake] Disable `openmp` as LLVM_ENABLE_PROJECTS (PR #110152)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

> What is the reason? Usually `libomp` doesn't need to be in runtimes.

On AIX, building `openmp` as a LLVM project never worked. It failed due to some non-standard code in AIX header files. Even though we believe building `libomp` should always be part of the runtimes, we only disable it for AIX.

https://github.com/llvm/llvm-project/pull/110152