LoongArch: Put the body of play_dead() into arch_cpu_idle_dead()

The initial aim is to silence the following objtool warning:

arch/loongarch/kernel/process.o: warning: objtool: arch_cpu_idle_dead() falls through to next function start_thread()

According to tools/objtool/Documentation/objtool.txt, this is because
the last instruction of arch_cpu_idle_dead() is a call to a noreturn
function play_dead(). In order to silence the warning, one simple way
is to add the noreturn function play_dead() to objtool's hard-coded
global_noreturns array, that is to say, just put "NORETURN(play_dead)"
into tools/objtool/noreturns.h, it works well.

But I noticed that play_dead() is only defined once and only called by
arch_cpu_idle_dead(), so put the body of play_dead() into the caller
arch_cpu_idle_dead(), then remove the noreturn function play_dead() is
an alternative way which can reduce the overhead of the function call
at the same time.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/smp.h

@@ -98,8 +98,6 @@ static inline void __cpu_die(unsigned int cpu)
 {
 	loongson_cpu_die(cpu);
 }
-
-extern void __noreturn play_dead(void);
 #endif
 
 #endif /* __ASM_SMP_H */

arch/loongarch/kernel/process.c

@@ -61,13 +61,6 @@ EXPORT_SYMBOL(__stack_chk_guard);
 unsigned long boot_option_idle_override = IDLE_NO_OVERRIDE;
 EXPORT_SYMBOL(boot_option_idle_override);
 
-#ifdef CONFIG_HOTPLUG_CPU
-void __noreturn arch_cpu_idle_dead(void)
-{
-	play_dead();
-}
-#endif
-
 asmlinkage void ret_from_fork(void);
 asmlinkage void ret_from_kernel_thread(void);
 

arch/loongarch/kernel/smp.c

@@ -317,7 +317,7 @@ void loongson_cpu_die(unsigned int cpu)
 	mb();
 }
 
-void play_dead(void)
+void __noreturn arch_cpu_idle_dead(void)
 {
 	register uint64_t addr;
 	register void (*init_fn)(void);