target/riscv: Factor out extension tests to cpu_cfg.h

target/riscv: Factor out extension tests to cpu_cfg.h

This patch moves the extension test functions that are used
to gate vendor extension decoders, into cpu_cfg.h.
This allows to reuse them in the disassembler.

This patch does not introduce new functionality.
However, the patch includes a small change:
The parameter for the extension test functions has been changed
from 'DisasContext*' to 'const RISCVCPUConfig*' to keep
the code in cpu_cfg.h self-contained.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Message-Id: <20230612111034.3955227-3-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>