riscv: Drop const annotation for sp

The const annotation should not be used for 'sp', or it will
become read only and lead to bad stack output.

Fixes: dec82277 ("riscv: stacktrace: Move register keyword to beginning of declaration")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>