Omit a test on s390x linux tripping over a git bug

This test has been reliably failing on recent trunk versions.

See: <https://github.com/ruby/ruby/actions/runs/19519712433/job/55880266450#step:14:120>

tool/test/test_sync_default_gems.rb

@@ -319,6 +319,9 @@ module Test_SyncDefaultGems
     end
 
     def test_squash_merge
+      if RUBY_PLATFORM =~ /s390x/
+        omit("git 2.43.0 bug on s390x ubuntu 24.04: BUG: log-tree.c:1058: did a remerge diff without remerge_objdir?!?")
+      end
       #   2---.   <- branch
       #  /     \
       # 1---3---3'<- merge commit with conflict resolution