mark noreturn functions

it's not guaranteed that all implementations of `reboot()` are
terminating calls.

`reboot_os()`, externally defined at
`src/arch/{x86_64,i686}/apic_asm.asm` both terminate, but this seems to
be an implementation detail of the x86 architecture.

Author: mazunki

api/arch.hpp

@@ -27,8 +27,8 @@
 #include <string>
 
 
-extern void __arch_poweroff();
-extern void __arch_reboot();
+[[noreturn]] extern void __arch_poweroff();
+[[noreturn]] extern void __arch_reboot();
 extern void __arch_enable_legacy_irq(uint8_t);
 extern void __arch_disable_legacy_irq(uint8_t);
 extern void __arch_system_deactivate();

api/os.hpp

@@ -86,7 +86,7 @@ namespace os {
   //
 
   /** Trigger unrecoverable error and output diagnostics **/
-  __attribute__((noreturn))
+  [[noreturn]]
   void panic(const char* why) noexcept;
 
   /** Default behavior after panic **/

src/kernel/syscalls.cpp

@@ -118,7 +118,7 @@ extern kernel::ctor_t __plugin_ctors_end;
  * Print EOT character to stderr, to signal outside that PANIC output completed
  * If the handler returns, go to (permanent) sleep
 **/
-void os::panic(const char* why) noexcept
+[[noreturn]] void os::panic(const char* why) noexcept
 {
   cpu_enable_panicking();
   if (kernel::panics() > 4) double_fault(why);

src/platform/x86_pc/acpi.cpp

@@ -393,8 +393,7 @@ namespace x86 {
     }
   }
 
-  __attribute__((noreturn))
-  void ACPI::shutdown()
+  [[noreturn]] void ACPI::shutdown()
   {
     asm("cli");
 

src/platform/x86_pc/acpi.hpp

@@ -102,7 +102,7 @@ namespace x86 {
     }
 
     static void reboot();
-    static void shutdown();
+    [[noreturn]] static void shutdown();
 
   private:
     void    discover();

src/platform/x86_pc/platform.cpp

@@ -147,12 +147,11 @@ void __arch_disable_legacy_irq(uint8_t irq)
   x86::APIC::disable_irq(irq);
 }
 
-void __arch_poweroff()
+[[noreturn]] void __arch_poweroff()
 {
   x86::ACPI::shutdown();
-  __builtin_unreachable();
 }
-void __arch_reboot()
+[[noreturn]] void __arch_reboot()
 {
   x86::ACPI::reboot();
   __builtin_unreachable();