Add support for B instructions

B is the bit-manipulation ISA extension, ratified into 2021: https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf

The specs is also available there: https://github.com/riscv/riscv-isa-manual/blob/main/src/b-st-ext.adoc

And here: https://drive.google.com/file/d/1uviu1nH-tScFfgrovvFCrj7Omv8tFtkp/view (chapter 29)