treewide: Fix Meson CPU names for powerpc CPUs (backport #13514)

[mergify]

(Still building on my machine, will undraft when it finishes & builtins.currentSystem returns the correct value)

Motivation

Meson's cpu_family() function returns ppc* on POWER systems, while Nixpkgs expects the previously-used powerpc* to be the CPU name:

puna@HC ~/D/nixpkgs (wip/ppc64-elfv1)> nix repl --file .
Nix 2.28.4
Type :? for help.
error:
       … while evaluating a branch condition
         at /home/puna/Development/nixpkgs/pkgs/stdenv/booter.nix:68:9:
           67|         pred: n:
           68|         if n == len then
             |         ^
           69|           rnul pred

       … while calling the 'length' builtin
         at /home/puna/Development/nixpkgs/pkgs/stdenv/booter.nix:65:13:
           64|     let
           65|       len = builtins.length list;
             |             ^
           66|       go =

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: Unknown CPU type: ppc64

Fix this discrepancy.

Context

Closes #12769

No real experience with Meson, so happy about better-looking suggestions.

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This is an automatic backport of pull request #13514 done by [Mergify](https://mergify.com).