re-introduce RUBY_GC_STRESS test removed by YARV.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12549 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

signal.c

@@ -1047,10 +1047,14 @@ Init_signal(void)
 #endif
 
 #ifdef SIGBUS
+# ifndef RUBY_GC_STRESS
     install_sighandler(SIGBUS, sigbus);
+# endif
 #endif
 #ifdef SIGSEGV
+# ifndef RUBY_GC_STRESS
     install_sighandler(SIGSEGV, sigsegv);
+# endif
 #endif
 #ifdef SIGPIPE
     install_sighandler(SIGPIPE, sigpipe);