arch-riscv: Don't crash when printing unknown CSRs

arch-riscv: Don't crash when printing unknown CSRs

This patch fixes a potential crash if an unnamed CSR is accessed and
debug flags are enabled that print disassembly.  Unknown CSRs will be
identified as "??" followed by the address that was used.

Change-Id: If5ac57f1422bd59c72a1a06206fa9d9dc05d21ef
Reviewed-on: https://gem5-review.googlesource.com/7321
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>