Zicond: implement Zicond (conditional integer operations)

This implements the Zicond (conditional integer operations) extension,
as of version 1.0-draft-20230120.

The Zicond extension acts as a building block for branchless sequences
including conditional-arithmetic, conditional-logic and
conditional-select/move.
The following instructions constitute Zicond:
 - czero.eqz rd, rs1, rs2  =>  rd = (rs2 == 0) ? 0 : rs1
 - czero.nez rd, rs1, rs2  =>  rd = (rs2 != 0) ? 0 : rs1

See
  https://github.com/riscv/riscv-zicond/releases/download/v1.0-draft-20230120/riscv-zicond_1.0-draft-20230120.pdf
for the proposed specification and usage details.