riscv: stack: Fixup independent irq stack for CONFIG_FRAME_POINTER=n

The independent irq stack uses s0 to save & restore sp, but s0 would be
corrupted when CONFIG_FRAME_POINTER=n. So add s0 in the clobber list to
fix the problem.

Fixes: 163e76cc

 ("riscv: stack: Support HAVE_IRQ_EXIT_ON_IRQ_STACK")
Cc: stable@vger.kernel.org
Reported-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Tested-by: Drew Fustini <dfustini@baylibre.com>
Link: https://lore.kernel.org/r/20230716001506.3506041-2-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>