systemd: be explicit about architecture support

systemd is not portable across all architectures.  It will only built
for architectures it recognises.  Listing them explicitly means
availableOn checks will work right for unsupported architectures, in
the way that's easiest to keep up to date as it can easily be compared
to the list in systemd's source code.

pkgs/os-specific/linux/systemd/default.nix

@@ -829,7 +829,9 @@ stdenv.mkDerivation (finalAttrs: {
       publicDomain
     ];
     maintainers = with lib.maintainers; [ flokli kloenk ];
-    platforms = lib.platforms.linux;
+    # See src/basic/missing_syscall_def.h
+    platforms = with lib.platforms; lib.intersectLists linux
+      (aarch ++ x86 ++ loongarch64 ++ m68k ++ mips ++ power ++ riscv ++ s390);
     priority = 10;
     badPlatforms = [
       # https://github.com/systemd/systemd/issues/20600#issuecomment-912338965