riscv/purgatory: Disable CFI

Filter out CC_FLAGS_CFI when CONFIG_CFI_CLANG.

Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20230710183544.999540-13-samitolvanen@google.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>