Make Time friendly to Ractor

[kirs]

(copy of ruby/ruby#4008)

Before this fix,

r = Ractor.new do
  Time.rfc2822("Fri, 21 Nov 1997 09:55:06 -0600")
end
Ractor.select(r)

fails with:

#<Thread:0x00007fe96306b810 run> terminated with exception (report_on_exception is true):
/opt/rubies/3.0.0/lib/ruby/3.0.0/time.rb:515:in `rfc2822': can not access non-shareable objects in constant #<Class:0x00007fe96408dfb8>::MonthValue by non-main ractor. (Ractor::IsolationError)
	from notractor.rb:14:in `block in <main>'
<internal:ractor>:345:in `select': thrown by remote Ractor. (Ractor::RemoteError)
	from notractor.rb:16:in `<main>'
/opt/rubies/3.0.0/lib/ruby/3.0.0/time.rb:515:in `rfc2822': can not access non-shareable objects in constant #<Class:0x00007fe96408dfb8>::MonthValue by non-main ractor. (Ractor::IsolationError)
	from notractor.rb:14:in `block in <main>'

@nobu @akr