stdenv: aarch64-linux: gcc9 -> gcc12

Hydra job building them: https://hydra.nixos.org/build/208909151

The bootstrap files can be reproduced on the commit 21ec906, e.g. by:
  cat $(nix-build pkgs/top-level/release.nix -QA stdenvBootstrapTools.aarch64-linux.dist)/nix-support/hydra-build-products
    file tarball /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/bootstrap-tools.tar.xz
    file busybox /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/busybox
and the hashes as well:
  nix hash file /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/bootstrap-tools.tar.xz
    sha256-aJvtsWeuQHbb14BGZ2EiOKzjQn46h3x3duuPEawG0eE=
  nix hash path /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/busybox
    sha256-0MuIeQlBUaeisqoFSu8y+8oB6K4ZG5Lhq8RcS9JqkFQ=

You can check this on any machine, as the builds are on cache.nixos.org
but also you can reproduce the hashes when rebuilt on aarch64-linux HW.

pkgs/stdenv/linux/bootstrap-files/aarch64.nix

@@ -1,11 +1,11 @@
 {
   busybox = import <nix/fetchurl.nix> {
-    url = "http://tarballs.nixos.org/stdenv-linux/aarch64/c7c997a0662bf88264db52cbc41e67884eb7a1ff/busybox";
-    sha256 = "sha256-4EN2vLvXUkelZZR2eKaAQA5kCEuHNvRZN6dcohxVY+c=";
+    url = "http://tarballs.nixos.org/stdenv-linux/aarch64/21ec906463ea8f11abf3f9091ddd4c3276516e58/busybox";
     executable = true;
+    hash = "sha256-0MuIeQlBUaeisqoFSu8y+8oB6K4ZG5Lhq8RcS9JqkFQ=";
   };
   bootstrapTools = import <nix/fetchurl.nix> {
-    url = "http://tarballs.nixos.org/stdenv-linux/aarch64/c7c997a0662bf88264db52cbc41e67884eb7a1ff/bootstrap-tools.tar.xz";
-    sha256 = "sha256-AjOvmaW8JFVZaBSRUMKufr9kJozg/tsZr7PvUEBQyi4=";
+    url = "http://tarballs.nixos.org/stdenv-linux/aarch64/21ec906463ea8f11abf3f9091ddd4c3276516e58/bootstrap-tools.tar.xz";
+    hash = "sha256-aJvtsWeuQHbb14BGZ2EiOKzjQn46h3x3duuPEawG0eE=";
   };
 }

pkgs/top-level/all-packages.nix

@@ -14289,7 +14289,6 @@ with pkgs;
   inherit (let
       num =
         if (with stdenv.targetPlatform; isVc4 || libc == "relibc") then 6
-        else if (stdenv.targetPlatform.isAarch64 && stdenv.isLinux) then 9
         else 12;
       numS = toString num;
     in {