by fixing optimizer bug with help of 'volatile' keyword


mysql-test/r/func_math.result:
  added tests for BUG #2338 Trigonometric arithmetic problems
mysql-test/t/func_math.test:
  added tests for BUG #2338 Trigonometric arithmetic problems
sql/item_func.cc:
  added keyword volatile in Item_func_acos::val and Item_func_asin::val
  to calm optimizer down and to avoid it's bug