riscv: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

Move POSIX CPU timer expiry and signal delivery into task context to
allow PREEMPT_RT setups to coexist with KVM.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>