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

[Kelvin Li via llvm-commits]

kkwli wrote:

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

I will take a look of it.

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