signal.c: SIGEXIT is not a system signal

* signal.c (trap): SIGEXIT is not a system signal and is dealt
  with internally, so it should not try to register the system
  signal handler by sigaction.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47669 b2dd03c8-39d4-4d8f-98ff-823fe69b080e