riscv: add PREEMPT_AUTO support

riscv has switched to GENERIC_ENTRY, so adding PREEMPT_AUTO is as simple
as adding TIF_ARCH_RESCHED_LAZY related definitions and enabling
HAVE_PREEMPT_AUTO.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>