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

[Peter Klausler via cfe-commits]

klausler 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

Capturing the results of a discussion elsewhere, mapping from `/dev/zero` when `MAP_ANON(YMOUS)` are undefined should solve the problem; might even be more portable.

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