tools/nolibc: use pselect6 on RISCV

[ Upstream commit 9c2970fb

 ]

This arch doesn't provide the old-style select() syscall, we have to
use pselect6().

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Stable-dep-of: 184177c3

 ("tools/nolibc: restore mips branch ordering in the _start block")
Signed-off-by: Sasha Levin <sashal@kernel.org>