[clang] [compiler-rt] [AIX] Undefine __STDC_NO_ATOMICS__ to enable c11 atomics functionality (PR #76025)

[David Tenty via cfe-commits]

https://github.com/daltenty requested changes to this pull request.

Last time I tested this functionality, we had some deadlocks if the standard library interfaces were used, which was part of the motivation for leaving this macro in.

It's possible the situation has changed since, but I'd like to confirm that first before we go ahead with removing the macro.

https://github.com/llvm/llvm-project/pull/76025