target/riscv: Add step to validate 'B' extension

target/riscv: Add step to validate 'B' extension

If the B extension is enabled warn if the user has disabled any of the
required extensions that are part of the 'B' extension. Conversely
enable the extensions that make up the 'B' extension if it is enabled.

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: <20240111161644.33630-3-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>