[buildtools] skip fetching GN on ppc and s390 platforms

GN is not available as a cipd package for ppc/s390 and
needs to be built from source.

Change-Id: I5f6eda13cd6227d20fc800cab7f54496a2d33f68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3663154Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#80743}

DEPS

@@ -104,7 +104,7 @@ deps = {
       }
     ],
     'dep_type': 'cipd',
-    'condition': 'host_os == "linux"',
+    'condition': 'host_os == "linux" and host_cpu != "s390" and host_cpu != "ppc"',
   },
   'buildtools/mac': {
     'packages': [