fix 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

rustsbi-qemu/src/main.rs

@@ -72,8 +72,8 @@ extern "C" fn rust_main(hartid: usize, opaque: 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);