tools/nolibc: Make getpgrp() fall back to getpgid(0)

The getpgrp() syscall is not implemented on arm64, so this commit instead
uses getpgid(0) when getpgrp() is not available.  This is a port of
nolibc's upstream commit 2379f25073f9 to the Linux kernel.

Fixes: 66b6f75 ("rcutorture: Import a copy of nolibc")
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/nolibc.h

@@ -1544,10 +1544,16 @@ int sys_getdents64(int fd, struct linux_dirent64 *dirp, int count)
 	return my_syscall3(__NR_getdents64, fd, dirp, count);
 }
 
+static __attribute__((unused))
+pid_t sys_getpgid(pid_t pid)
+{
+	return my_syscall1(__NR_getpgid, pid);
+}
+
 static __attribute__((unused))
 pid_t sys_getpgrp(void)
 {
-	return my_syscall0(__NR_getpgrp);
+	return sys_getpgid(0);
 }
 
 static __attribute__((unused))
@@ -1950,6 +1956,18 @@ int getdents64(int fd, struct linux_dirent64 *dirp, int count)
 	return ret;
 }
 
+static __attribute__((unused))
+pid_t getpgid(pid_t pid)
+{
+	pid_t ret = sys_getpgid(pid);
+
+	if (ret < 0) {
+		SET_ERRNO(-ret);
+		ret = -1;
+	}
+	return ret;
+}
+
 static __attribute__((unused))
 pid_t getpgrp(void)
 {