lib: remove dependency on feature asm_sym

It has been stablized in rust-lang/rust#103168

Author: luojia65

README.md

@@ -119,6 +119,10 @@ You should use these following line of parameters:
 
 ... to enable H extension on QEMU software.
 
+3. What is the minimum supported Rust version of this package?
+
+You should build RustSBI-QEMU on nightly at least `rustc 1.66.0-nightly (a24a020e6 2022-10-18)`.
+
 ## License 
 
 This project is licensed under Mulan PSL v2.

rustsbi-qemu/src/main.rs

@@ -1,6 +1,6 @@
 #![no_std]
 #![no_main]
-#![feature(naked_functions, asm_sym, asm_const)]
+#![feature(naked_functions, asm_const)]
 #![deny(warnings)]
 
 mod clint;

test-kernel/src/main.rs

@@ -1,6 +1,6 @@
 #![no_std]
 #![no_main]
-#![feature(naked_functions, asm_sym, asm_const)]
+#![feature(naked_functions, asm_const)]
 #![deny(warnings)]
 
 #[macro_use]