Resurrect gem-specific patterns for sync

Yet another partial revert of https://github.com/ruby/ruby/pull/8329,
similar to 00f263e6c4.

Repro: On ruby/ruby@1be64e34d0, `tool/sync_default_gems.rb yarp
162c2088eec6ec8f0558559e082cd661c18ee02a` should exist successfully, but
it doesn't without this gem-specific handling.

tool/sync_default_gems.rb

@@ -447,7 +447,15 @@ module SyncDefaultGems
 
     # Gem-specific patterns
     case gem
-    when nil
+    when "yarp"
+      %r[\A(?:
+        Makefile\.in
+        |configure\.ac
+        |fuzz/.*
+        |rust/.*
+        |tasks/.*
+        |ext/yarp/extconf\.rb
+      )\z]mx
     end&.tap do |pattern|
       patterns << pattern
     end