x86: Mark stop_this_cpu() __noreturn

vmlinux.o: warning: objtool: smp_stop_nmi_callback()+0x2b: unreachable instruction

0000 0000000000047cf0 <smp_stop_nmi_callback>:
...
0026    47d16:  e8 00 00 00 00          call   47d1b <smp_stop_nmi_callback+0x2b>       47d17: R_X86_64_PLT32   stop_this_cpu-0x4
002b    47d1b:  b8 01 00 00 00          mov    $0x1,%eax

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.290905453@infradead.org

arch/x86/include/asm/processor.h

@@ -835,7 +835,7 @@ bool xen_set_default_idle(void);
 #define xen_set_default_idle 0
 #endif
 
-void stop_this_cpu(void *dummy);
+void __noreturn stop_this_cpu(void *dummy);
 void microcode_check(void);
 
 enum l1tf_mitigations {

arch/x86/kernel/process.c

@@ -747,7 +747,7 @@ bool xen_set_default_idle(void)
 }
 #endif
 
-void stop_this_cpu(void *dummy)
+void __noreturn stop_this_cpu(void *dummy)
 {
 	local_irq_disable();
 	/*

tools/objtool/check.c

@@ -181,6 +181,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"kunit_try_catch_throw",
 		"xen_start_kernel",
 		"cpu_bringup_and_idle",
+		"stop_this_cpu",
 	};
 
 	if (!func)