target/riscv: support disable auto fence

[zqb-all]

Support disable automatic fence, it's useful for debug some cache related issue.

[aap-sc]

@zqb-all I'm not sure we need the functionality of riscv fence command, since we already have capabilities to execute instructions from program buffer directly. Have you tried riscv exec_progbuf command?

[aap-sc]

as for auto fence - it looks reasonable

[zqb-all]

@zqb-all I'm not sure we need the functionality of riscv fence command, since we already have capabilities to execute instructions from program buffer directly. Have you tried riscv exec_progbuf command?

Thanks, I didn't notice this command before. With this command, I agree that a separate fence command is not needed