libc: Implement getppid

as specified here:
https://pubs.opengroup.org/onlinepubs/009695399/functions/getppid.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

include/sys/syscall_lookup.h

@@ -28,6 +28,10 @@ SYSCALL_LOOKUP1(_exit,                     1)
 SYSCALL_LOOKUP(exit,                       1)
 SYSCALL_LOOKUP(getpid,                     0)
 
+#ifdef CONFIG_SCHED_HAVE_PARENT
+  SYSCALL_LOOKUP(getppid,                  0)
+#endif
+
 SYSCALL_LOOKUP(sched_getparam,             2)
 SYSCALL_LOOKUP(sched_getscheduler,         1)
 SYSCALL_LOOKUP(sched_lock,                 0)

include/unistd.h

@@ -315,6 +315,9 @@ EXTERN int       optopt; /* Unrecognized option character */
 pid_t   vfork(void);
 pid_t   getpid(void);
 pid_t   gettid(void);
+#ifdef CONFIG_SCHED_HAVE_PARENT
+pid_t   getppid(void);
+#endif
 void    _exit(int status) noreturn_function;
 unsigned int sleep(unsigned int seconds);
 int     usleep(useconds_t usec);