The bc022f8cd8 introduces a static way to calculate idle stack
address for risc-v platform. However, it uses the reverse order
to access idle stack, which breaks boards with smp configuration.
Correct the idle stack order of g_cpux_idlestack.
Fixes: bc022f8cd8 ("arch/risc-v: unify idle stack calculation")
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>