[AVR] Fix debug printing of function pointers

This commit fixes debug printing of function pointers on AVR. AVR does
not support `addrspacecast` instructions, and so this patch modifies
libcore so that a `ptrtoint` IR instruction is used and the address
space cast is avoided.

Author: dylanmckay

src/libcore/ptr/mod.rs

@@ -2836,14 +2836,14 @@ 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)
+                fmt::Pointer::fmt(&(*self as usize as *const ()), 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)
+                fmt::Pointer::fmt(&(*self as usize as *const ()), f)
             }
         }
     }