[rtl] Consider implementation of the code size reduction extension (Zce)

[GregAC]

The code size reduction extension was recently released: https://github.com/riscv/riscv-code-size-reduction/releases/tag/v1.0

For Ibex we can consider implementing the Zce subset. Some of these introduce new 16-bit encodings for existing instructions so work required will be minimal. Others are more complex instructions (e.g. cm.push and cm.pop) requiring more work (in particular care will be needed for DV as this will introduce one or more new classes of multi-cycle instructions).

The Zcb subset (itself part of Zce) implements sign and zero extend operations that can be implemented with existing bitmanip functionality, however we may wish to support a configuration enabling Zce without requiring bitmanip in full.