Add Zicfiss extension

[ved-rivos]

Specification: https://github.com/riscv/riscv-cfi/blob/main/cfi_backward.adoc Tests: https://github.com/riscv-non-isa/riscv-arch-test/pull/417

Includes the unratified Zimop and Zcmop extensions which Zicfiss depends on: https://github.com/riscv/riscv-isa-manual/blob/main/src/zimop.adoc

Parts of the sw-check exception code is duplicated here and in PR #361

[Alasdair]

Just letting you know I am away on Holiday until early January, so it might take me a while before I can go through this in detail.

[github-actions[bot]]

Test Results

712 tests  ±0   712 :white_check_mark: ±0   0s :stopwatch: ±0s   6 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit 16150636. ± Comparison against base commit 40ed0c5b.

:recycle: This comment has been updated with latest results.