Environment

Provide at least:

• JRuby version: jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 Java HotSpot(TM) 64-Bit Server VM 25.181-b13 on 1.8.0_181-b13 +jit [darwin-x86_64]
• Operating system and platform (e.g. uname -a): Darwin MB-ADPENTELA.local 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64
  Other relevant info you may wish to add:
• Installed or activated gems
• Application/framework version (e.g. Rails, Sinatra)
• Environment variables

Expected Behavior

• The Signal.trap and at_exit hooks are executed in different threads leading to a race-condition where both at_exit and Signal.trap are defined.

Example: Puma traps SIGINT and sucker_punch uses the at_exit hook leading to a SystemExit before sucker_punch workers are shutdown.
puma/puma#1675
brandonhilkert/sucker_punch#223

Actual Behavior

MRI is using the same thread for both at_exit and Signal.trap. It would be nice if there is some synchronization.

### test.rb
at_exit do
  sleep(1)
  puts "Thread in at_exit: #{Thread.current}"
end

Signal.trap("SIGINT") do
 puts "Thread in trap: #{Thread.current}"
 exit
end

sleep

## JRuby
$ RUBYOPT=-d jruby test_in_jruby.rb 
Exception `Gem::MissingSpecError' at /Users/adpentela/.rvm/rubies/jruby-9.2.0.0/lib/ruby/stdlib/rubygems/dependency.rb:310 - Gem::MissingSpecError
^CThread in trap: #<Thread:0x19b33233>
Exception `SystemExit' at org/jruby/RubyKernel.java:735 - exit
Thread in at_exit: #<Thread:0x37d31475>

## MRI
 $ ruby test_in_jruby.rb 
^CThread in trap: #<Thread:0x00007fd94d085c40 run>
Thread in at_exit: #<Thread:0x00007fd94d085c40 run>

Let me know if I'm doing something wrong or if this is a duplicate issue.
I had a quick glance at #4956 but not sure if this is related.

Thanks!