[clang] [flang] [llvm] [flang] Add runtime trampoline pool for W^X compliance (PR #183108)

[Kelvin Li via llvm-commits]

kkwli wrote:

I get the following error on AIX.
```
/home/kli/llvm-project/flang-rt/lib/runtime/trampoline.cpp:227:23: error: use of undeclared identifier 'MAP_ANONYMOUS'
  227 |         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
      |                       ^~~~~~~~~~~~~
1 error generated.
```
We are looking into how to resolve it.
cc @DanielCChen 

https://github.com/llvm/llvm-project/pull/183108