Ensure x86 stack is fortified, fixed #15335.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

coroutine/x86/Context.S

@@ -37,3 +37,7 @@ _coroutine_transfer:
 
 	# Jump to the address on the stack
 	ret
+
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif