LibC: Mark _exit() as noreturn

We already do this for exit().

Libraries/LibC/stdlib.cpp

@@ -208,7 +208,6 @@ void exit(int status)
     fflush(stdout);
     fflush(stderr);
     _exit(status);
-    ASSERT_NOT_REACHED();
 }
 
 static void __atexit_to_cxa_atexit(void* handler)

Libraries/LibC/unistd.h

@@ -72,7 +72,7 @@ int execlp(const char* filename, const char* arg, ...);
 int chroot(const char* path);
 int chroot_with_mount_flags(const char* path, int mount_flags);
 void sync();
-void _exit(int status);
+__attribute__((noreturn)) void _exit(int status);
 pid_t getsid(pid_t);
 pid_t setsid();
 int setpgid(pid_t pid, pid_t pgid);