Avoid creating a Fiber while loading the gem

* Since creating a Fiber can be quite expensive
  and might not work on some platforms (though those are bugs):
  https://bugs.ruby-lang.org/issues/20085
  https://bugs.ruby-lang.org/issues/21607
* spec/concurrent/atomic/lock_local_var_spec.rb already checks
  that the shortcut is correct and matches the reality.

Author: eregon

lib/concurrent-ruby/concurrent/atomic/lock_local_var.rb

@@ -6,6 +6,7 @@ module Concurrent
   # @!visibility private
   def self.mutex_owned_per_thread?
     return false if Concurrent.on_jruby? || Concurrent.on_truffleruby?
+    return RUBY_VERSION < "3.0" if Concurrent.on_cruby?
 
     mutex = Mutex.new
     # Lock the mutex: