buildMozillaMach: use llvmPackages_20 on aarch64-darwin

There's a bug in llvm 21 that makes Firefox crash on loading a PNG
favicon that is being investigated in Mozilla bug tracker:

https://bugzilla.mozilla.org/show_bug.cgi?id=1995582

While this is being fixed, switch to llvmPackages_20 for aarch64-darwin.
(x86_64-darwin is not affected.)

Note: this change will also affect Thunderbird. Though strictly speaking
not a required change because we are not aware of a specific issue with
Thunderbird (yet), I feel we may want to stick to the same LLVM for both
types of Mozilla apps while this is being investigated. It also
simplifies the code somewhat.

Fixes #453372

pkgs/build-support/build-mozilla-mach/default.nix

@@ -58,9 +58,7 @@ in
   pkgsCross, # wasm32 rlbox
   python3,
   runCommand,
-  rustc,
   rust-cbindgen,
-  rustPlatform,
   unzip,
   which,
   wrapGAppsHook3,
@@ -201,9 +199,25 @@ assert elfhackSupport -> isElfhackPlatform stdenv;
 let
   inherit (lib) enableFeature;
 
+  rustPackages =
+    pkgs:
+    (pkgs.rust.override (
+      # aarch64-darwin firefox crashes on loading favicons due to a llvm 21 bug:
+      # https://github.com/NixOS/nixpkgs/issues/453372
+      # https://bugzilla.mozilla.org/show_bug.cgi?id=1995582#c16
+      lib.optionalAttrs (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64) {
+        llvmPackages = pkgs.llvmPackages_20;
+      }
+    )).packages.stable;
+
+  toRustC = pkgs: (rustPackages pkgs).rustc;
+
+  rustc = toRustC pkgs;
+  inherit (rustPackages pkgs) rustPlatform;
+
   # Target the LLVM version that rustc is built with for LTO.
   llvmPackages0 = rustc.llvmPackages;
-  llvmPackagesBuildBuild0 = pkgsBuildBuild.rustc.llvmPackages;
+  llvmPackagesBuildBuild0 = (toRustC pkgsBuildBuild).llvmPackages;
 
   # Force the use of lld and other llvm tools for LTO
   llvmPackages = llvmPackages0.override {
@@ -218,7 +232,7 @@ let
   # LTO requires LLVM bintools including ld.lld and llvm-ar.
   buildStdenv = overrideCC llvmPackages.stdenv (
     llvmPackages.stdenv.cc.override {
-      bintools = if ltoSupport then buildPackages.rustc.llvmPackages.bintools else stdenv.cc.bintools;
+      bintools = if ltoSupport then (toRustC buildPackages).llvmPackages.bintools else stdenv.cc.bintools;
     }
   );