Dependent extensions could be improved in several ways.

Currently, the dependent cpu extensions are processed in 2 separate places:
1. update_flag() when calling VM_Version::setup_cpu_available_features()
2. at the end of VM_Version::common_initialize().
But we can do it in one single place, that is update_flag().

And, dependency relationship can only be expressed in (1:1) way, can not be expressed (1:N) way, but in fact some extensions can depend on several different other extensions, for example, zvfh depends on zvf and rvv. It's would be better to support this (1:N) relationship.