target/riscv: Add infrastructure for 'B' MISA extension

target/riscv: Add infrastructure for 'B' MISA extension

Add the infrastructure for the 'B' extension which is the union of the
Zba, Zbb and Zbs instructions.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240111161644.33630-2-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>