Use `intrinsics!` macro for math intrinsics

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

src/math.rs

@@ -4,13 +4,14 @@ mod libm;
 
 macro_rules! no_mangle {
     ($(fn $fun:ident($($iid:ident : $ity:ty),+) -> $oty:ty;)+) => {
+        intrinsics! {
             $(
-            #[no_mangle]
+                pub extern "C" fn $fun($($iid: $ity),+) -> $oty {
-            pub fn $fun($($iid: $ity),+) -> $oty {
                     self::libm::$fun($($iid),+)
                 }
             )+
         }
+    }
 }
 
 no_mangle! {