RISC-V: Add a missing "," in riscv_excp_names

RISC-V: Add a missing "," in riscv_excp_names

This would almost certainly cause the exception names to be reported
incorrectly.  Coverity found the issue (CID 1420223).  As per Peter's
suggestion, I've also added a comma at the end of the list to avoid the issue
reappearing in the future.

Fixes: ab67a1d07a ("target/riscv: Add support for the new execption numbers")
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>