tools/nolibc: arm: add missing my_syscall6

tools/nolibc: arm: add missing my_syscall6

This is required by the coming removal of the oldselect and newselect
support.

pselect6/pselect6_time64 will be used unconditionally, they have 6
arguments.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/linux-riscv/bf3e07c1-75f5-425b-9124-f3f2b230e63a@app.fastmail.com/
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>