add a fallback implementation for the prefetch_* intrinsics

The fallback is to just ignore the arguments. That is a valid implementation because this intrinsic is just a hint.

I also added `miri::intrinsic_fallback_is_spec` annotation, so that miri now supports these operations. A prefetch intrinsic call is valid on any pointer.

Author: folkertdev

tests/pass/prefetch.rs

@@ -0,0 +1,23 @@
+#![feature(core_intrinsics)]
+
+fn main() {
+    static X: [u8; 8] = [0; 8];
+
+    unsafe {
+        ::std::intrinsics::prefetch_read_data(::std::ptr::null::<u8>(), 1);
+        ::std::intrinsics::prefetch_read_data(::std::ptr::dangling::<u8>(), 2);
+        ::std::intrinsics::prefetch_read_data(X.as_ptr(), 3);
+
+        ::std::intrinsics::prefetch_write_data(::std::ptr::null::<u8>(), 1);
+        ::std::intrinsics::prefetch_write_data(::std::ptr::dangling::<u8>(), 2);
+        ::std::intrinsics::prefetch_write_data(X.as_ptr(), 3);
+
+        ::std::intrinsics::prefetch_read_instruction(::std::ptr::null::<u8>(), 1);
+        ::std::intrinsics::prefetch_read_instruction(::std::ptr::dangling::<u8>(), 2);
+        ::std::intrinsics::prefetch_read_instruction(X.as_ptr(), 3);
+
+        ::std::intrinsics::prefetch_write_instruction(::std::ptr::null::<u8>(), 1);
+        ::std::intrinsics::prefetch_write_instruction(::std::ptr::dangling::<u8>(), 2);
+        ::std::intrinsics::prefetch_write_instruction(X.as_ptr(), 3);
+    }
+}