[flang] [llvm] [flang/flang-rt] Implement PERROR intrinsic form GNU Extension (PR #132406)

[Tom Eccles via llvm-commits]

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

Thanks for the update. LGTM.

nit: please could you add a test where the box_addr is not canonicalized away? I think using a string as a subroutine argument where the length is not known at compile time should work.

https://github.com/llvm/llvm-project/pull/132406