arm64: LLVMLinux: Use current_stack_pointer in save_stack_trace_tsk

Use the global current_stack_pointer to get the value of the stack pointer.
This change supports being able to compile the kernel with both gcc and clang.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Reviewed-by: Jan-Simon Möller <dl9pf@gmx.de>
Reviewed-by: Olof Johansson <olof@lixom.net>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/stacktrace.c

@@ -111,10 +111,9 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 		frame.sp = thread_saved_sp(tsk);
 		frame.pc = thread_saved_pc(tsk);
 	} else {
-		register unsigned long current_sp asm("sp");
 		data.no_sched_functions = 0;
 		frame.fp = (unsigned long)__builtin_frame_address(0);
-		frame.sp = current_sp;
+		frame.sp = current_stack_pointer;
 		frame.pc = (unsigned long)save_stack_trace_tsk;
 	}