arch-riscv: Update CSR implementations

This patch updates the CSRs to match the RISC-V privileged specification
version 1.10. As interrupts, faults, and privilege levels are not yet
supported, there are no meaninful side effects that are implemented.
Performance counters are also not yet implemented, as they do not have
specifications. Currently they act as cycle counters.

Note that this implementation trusts software to use the registers
properly. Access protection, readability, and writeability of registers
based on privilege will come in a future patch.

Change-Id: I1de89bdbe369b5027911b2e6bc0425d3acaa708a
Reviewed-on: https://gem5-review.googlesource.com/7441
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>

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>

arch-riscv: Move standard ops out of ISA

This patch removes static portions of the standard instruction types
from the generated ISA code and puts them into arch/riscv/insts. Some
dynamically-generated content is left behind for each individual
instruction's implementation. Also, BranchOp is removed due to its
similarity with ImmOp and ImmOp and UImmOp are joined into a single
templated class, ImmOp<T>.

Change-Id: I1bf47c8b8a92a5be74a50909fcc51d8551185a2a
Reviewed-on: https://gem5-review.googlesource.com/6022
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>