C functions can be tail called, too

A tail call to a C function can have the behavior of a "real" tail
call, reusing the stack frame of the caller.

testes/coroutine.lua

@@ -205,7 +205,7 @@ do
   co = coroutine.create(function () return pcall(foo) end)
   local st1, st2, err = coroutine.resume(co)
   assert(st1 and not st2 and err == 43)
-  assert(X == 43 and Y.name == "pcall")
+  assert(X == 43 and Y.what == "C")
 
   -- recovering from errors in __close metamethods
   local track = {}