riscv: allow resume after exception

If CSRs like seed are readable by S-mode, may not be determinable by
S-mode. For safe driver probing allow to resume via a longjmp after an
exception.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>