Implement `Zbkx`

[TheDeepestSpace]

Spec: https://docs.riscv.org/reference/isa/v20260120/unpriv/b-st-ext.html#zbkx

This is a sub-extension with just two operations -- xperm4 and xperm8 ; operation detials are in the spec.

To do:

• Extend the decoder to support these two operations
• Implement the sub-extension ALU in src/ext/b/zbkx.sv per specification
• Integrate the ALU into the execute stage, gated by the existing B extension feature flag (see src/stages/execute_stage.sv)
• Test the extension by enabling in in RISCOF