Changed altivec.rs to new intrinsic declaration

Author: aaishwarymishra

crates/core_arch/src/powerpc/altivec.rs

@@ -688,10 +688,13 @@ mod sealed {
                 let addr = (b as *const u8).offset(a);
 
                 // Workaround ptr::copy_nonoverlapping not being inlined
-                unsafe extern "rust-intrinsic" {
+                    #[rustc_intrinsic]
+                    #[rustc_intrinsic_must_be_overridden]
                     #[rustc_nounwind]
-                    pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
-                }
+                    pub fn copy_nonoverlapping<T>(_src: *const T, _dst: *mut T, _count: usize) {
+                        unreachable!()
+                    }
+                
 
                 let mut r = mem::MaybeUninit::uninit();
 
@@ -743,10 +746,13 @@ mod sealed {
                 let addr = (b as *mut u8).offset(a);
 
                 // Workaround ptr::copy_nonoverlapping not being inlined
-                unsafe extern "rust-intrinsic" {
+                    #[rustc_intrinsic]
+                    #[rustc_intrinsic_must_be_overridden]
                     #[rustc_nounwind]
-                    pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
-                }
+                    pub fn copy_nonoverlapping<T>(_src: *const T, _dst: *mut T, _count: usize) {
+                        unreachable!()
+                    }
+                
 
                 copy_nonoverlapping(
                     &s as *const _ as *const u8,