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 <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Josh Poimboeuf <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/cpu.h

···
 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

···
 		"do_group_exit",
 		"stop_this_cpu",
 		"__invalid_creds",
+               "cpu_startup_entry",
 	};
 
 	if (!func)