lib: remove dependency on feature asm_sym

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

Author: luojia65

see/src/main.rs

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

spl/src/main.rs

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

test-kernel/src/main.rs

@@ -2,7 +2,7 @@
 
 #![no_std]
 #![no_main]
-#![feature(naked_functions, asm_sym, asm_const)]
+#![feature(naked_functions, asm_const)]
 
 use core::arch::asm;
 use sbi_testing::sbi;