selftests/riscv: Add Zicbop prefetch test

selftests/riscv: Add Zicbop prefetch test

Add selftests to cbo.c to verify Zicbop extension behavior, and split
the previous `--sigill` mode into two options so they can be tested
independently.

The test checks:
- That hwprobe correctly reports Zicbop presence and block size.
- That prefetch instructions execute without exception on valid and NULL
  addresses when Zicbop is present.

Signed-off-by: Yao Zihong <zihong.plct@isrc.iscas.ac.cn>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://patch.msgid.link/20251118162436.15485-3-zihong.plct@isrc.iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>