[AVR][HACK][NO UPSTREAM] Disable debug printing of function pointers

Workaround for #143.

Author: dylanmckay

src/libcore/ptr.rs

@@ -2624,14 +2624,14 @@ macro_rules! fnptr_impls_safety_abi {
         #[stable(feature = "fnptr_impls", since = "1.4.0")]
         impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                fmt::Pointer::fmt(&(*self as *const ()), f)
+                "disabled due to avr-rust/rust#143".fmt(f)
             }
         }
 
         #[stable(feature = "fnptr_impls", since = "1.4.0")]
         impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                fmt::Pointer::fmt(&(*self as *const ()), f)
+                "disabled due to avr-rust/rust#143".fmt(f)
             }
         }
     }