target/riscv: Remove experimental prefix from "B" extension

target/riscv: Remove experimental prefix from "B" extension

This extension has now been ratified:
https://jira.riscv.org/browse/RVS-2006 so the "x-" prefix can be
removed.

Since this is now a ratified extension add it to the list of extensions
included in the "max" CPU variant.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20240514110217.22516-1-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>