[libc] Mark cast_to as always_inline

I've seen the compiler emit a call to cast_to, which it should never do.

src/libraries/libc/__j6libc/casts.h

@@ -31,6 +31,7 @@ inline T cast_to(S p) { return reinterpret_cast<T>(p); }
 */
 
 template <typename T, typename S>
+__attribute__ ((always_inline))
 inline T cast_to(S p) {
     return reinterpret_cast<T>(const_cast<typename non_const<S>::type>(p));
 }