x86,objtool: Mark cpu_startup_entry() __noreturn

GCC-8 isn't clever enough to figure out that cpu_start_entry() is a
noreturn while objtool is. This results in code after the call in
start_secondary(). Give GCC a hand so that they all agree on things.

  vmlinux.o: warning: objtool: start_secondary()+0x10e: unreachable
Reported-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220408094718.383658532@infradead.org

include/linux/cpu.h

@@ -167,7 +167,7 @@ static inline int suspend_disable_secondary_cpus(void) { return 0; }
 static inline void suspend_enable_secondary_cpus(void) { }
 #endif /* !CONFIG_PM_SLEEP_SMP */
 
-void cpu_startup_entry(enum cpuhp_state state);
+void __noreturn cpu_startup_entry(enum cpuhp_state state);
 
 void cpu_idle_poll_ctrl(bool enable);
 

tools/objtool/check.c

@@ -184,6 +184,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"do_group_exit",
 		"stop_this_cpu",
 		"__invalid_creds",
+               "cpu_startup_entry",
 	};
 
 	if (!func)