vm_core.h: unreachable VM_UNREACHABLE

* vm_core.h (VM_UNREACHABLE): tell unreachable if possible.
  suppress an invalid-noreturn warning at rb_fiber_start.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59568 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -51,7 +51,7 @@
 
 #else
 #define VM_ASSERT(expr) ((void)0)
-#define VM_UNREACHABLE(func) ((void)0)
+#define VM_UNREACHABLE(func) UNREACHABLE
 #endif
 
 #define RUBY_VM_THREAD_MODEL 2