llvmPackages_16: Drop platforms.{power,s390x} from meta.platforms

This code is backported from nixos-unstable (2c627d9c70) and the power
platform isn't defined/supported on NixOS 22.11 yet, e.g.:
```
$ nix-instantiate -A llvmPackages_16
error: attribute 'power' missing

       at /home/michael/nixpkgs/worktrees/22.11/pkgs/development/compilers/llvm/16/default.nix:96:7:

           95|       lib.platforms.mips ++
           96|       lib.platforms.power ++
             |       ^
           97|       lib.platforms.riscv ++
(use '--show-trace' to show detailed location information)
```

pkgs/development/compilers/llvm/16/default.nix

@@ -93,9 +93,7 @@ in let
       lib.platforms.arm ++
       lib.platforms.m68k ++
       lib.platforms.mips ++
-      lib.platforms.power ++
       lib.platforms.riscv ++
-      lib.platforms.s390x ++
       lib.platforms.wasi ++
       lib.platforms.x86;
   };