riscv: stacktrace: Move register keyword to beginning of declaration

Using global sp_in_global directly to fix the following warning,

arch/riscv/kernel/stacktrace.c:31:3: warning: ‘register’ is not at beginning of declaration [-Wold-style-declaration]
31 |   const register unsigned long current_sp = sp_in_global;
   |   ^~~~~
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/stacktrace.c

@@ -14,7 +14,7 @@
 
 #include <asm/stacktrace.h>
 
-register unsigned long sp_in_global __asm__("sp");
+register const unsigned long sp_in_global __asm__("sp");
 
 #ifdef CONFIG_FRAME_POINTER
 
@@ -28,9 +28,8 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 		sp = user_stack_pointer(regs);
 		pc = instruction_pointer(regs);
 	} else if (task == NULL || task == current) {
-		const register unsigned long current_sp = sp_in_global;
 		fp = (unsigned long)__builtin_frame_address(0);
-		sp = current_sp;
+		sp = sp_in_global;
 		pc = (unsigned long)walk_stackframe;
 	} else {
 		/* task blocked in __switch_to */