[llvm] [flang] Set compile definitions for flang-rt build on AIX (PR #127919)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur approved this pull request.

Would this otherwise come from https://github.com/llvm/llvm-project/blob/0181af2f32e51c2ac45d96bd194884d7adfd10ca/llvm/CMakeLists.txt#L1189 ?

Besides such global configuration changes not conforming modern CMake, this didn't work for standalone Flang builds either. 

https://github.com/llvm/llvm-project/pull/127919