signal.c: prefer pthread_sigmask
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58361 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
signal.c: prefer pthread_sigmask
sigprocmask, for multithreading.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58361 b2dd03c8-39d4-4d8f-98ff-823fe69b080e