riscv: use 16KB kernel stack on 64-bit

With the current 8KB stack size there are frequent overflows in a 64-bit
configuration.  We may split IRQ stacks off in the future, but this fixes a
number of issues right now.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
[Palmer: mention irqstack in the commit text]
Fixes: 7db91e57 ("RISC-V: Task implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>