fix test module compile error with rustc 1.77.0-nightly (62d7ed4a6 2024-01-11)

mutable reference of mutable static is discouraged
for more information, see issue #114447 <rust-lang/rust#114447>
reference of mutable static is a hard error from 2024 edition

Author: wenhaozhao

test-kernel/src/main.rs

@@ -40,8 +40,8 @@ extern "C" fn rust_main(hartid: usize, dtb_pa: usize) -> ! {
         static mut ebss: u64;
     }
     unsafe {
-        let mut ptr = &mut sbss as *mut u64;
-        let end = &mut ebss as *mut u64;
+        let mut ptr = sbss as *mut u64;
+        let end = ebss as *mut u64;
         while ptr < end {
             ptr.write_volatile(0);
             ptr = ptr.offset(1);