Disable RISC-V A extension by default

[xxuejie]

While there is experimental support for A extension in CKB-VM, no CKB versions enable A extension for now. This commit fixes makefile so we are not generating any instruction from A extension by default, which would be a benefit for developers(since they don't have to do this by themselves)

[XuJiandong]

We will release ckb-std 0.16.0 with dummy-atomic as default feature in the following week.
After releasing, the dependency of ckb-std can be updated to 0.16.0.

[xxuejie]

To be honest I don't think the problem is ckb-std, it is actually molecule (which is typically used by ckb-gen-types). Even when someone uses an older version of ckb-std but upgrades ckb-gen-types or molecule, this could still be an issue.

I've updated the documentation with this change.