tools/nolibc: use pselect6 on RISCV

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>

tools/include/nolibc/nolibc.h

@@ -1256,7 +1256,10 @@ struct sys_stat_struct {
  *   - the arguments are cast to long and assigned into the target
  *     registers which are then simply passed as registers to the asm code,
  *     so that we don't have to experience issues with register constraints.
+ *
+ * On riscv, select() is not implemented so we have to use pselect6().
  */
+#define __ARCH_WANT_SYS_PSELECT6
 
 #define my_syscall0(num)                                                      \
 ({                                                                            \