Following how mjit_worker.c currently works, rb_f_system
now ensures the VM-wide waitpid lists is locked before
creating a new process via fork/vfork.

This ensures other rb_waitpid callers cannot steal work and
there are no possible race conditions from toggling
ruby_nocldwait without the use of atomics.

This sets us up for implementing MJIT process management
logic using normal Ruby APIs prepares us for VM-wide
asynchronous/event-base waitpid which can allow MJIT to
work without worker threads.

Take 2: set waitpid_state.pid on platforms w/o fork.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65437 b2dd03c8-39d4-4d8f-98ff-823fe69b080e