When commit 0643c12e dropped the 'x-' prefix for Zb[abcs] and set
them to be enabled by default, the comment about experimental
extensions was kept in place above them.  This moves it down a few
lines to only cover experimental extensions.

References: 0643c12e ("target/riscv: Enable bitmanip Zb[abcs] instructions")

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220106134020.1628889-1-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>