Use `intrinsics!` macro for math intrinsics

Make sure they respect the `mangled-names` feature as well as have the `"C"`
ABI.

src/math.rs

@@ -4,12 +4,13 @@ mod libm;
 
 macro_rules! no_mangle {
     ($(fn $fun:ident($($iid:ident : $ity:ty),+) -> $oty:ty;)+) => {
-        $(
+        intrinsics! {
-            #[no_mangle]
+            $(
-            pub fn $fun($($iid: $ity),+) -> $oty {
+                pub extern "C" fn $fun($($iid: $ity),+) -> $oty {
-                self::libm::$fun($($iid),+)
+                    self::libm::$fun($($iid),+)
-            }
+                }
-        )+
+            )+
+        }
     }
 }