bootstrap and compiletest: Use size_of_val from the prelude instead of imported

Use `std::mem::size_of_val` from the prelude instead of importing or
qualifying it.

This function was added to all preludes in Rust 1.80.

src/tools/compiletest/src/raise_fd_limit.rs

@@ -7,7 +7,6 @@
 #[cfg(target_vendor = "apple")]
 #[allow(non_camel_case_types)]
 pub unsafe fn raise_fd_limit() {
-    use std::mem::size_of_val;
     use std::ptr::null_mut;
     use std::{cmp, io};