riscv: ignore nanosleep syscall
riscv: ignore nanosleep syscall

Change-Id: I564a09564da668a5db3e75f15b33efaca363d71a
Reviewed-on: https://gem5-review.googlesource.com/c/9624
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5