Extract platform-related code from axhal

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Currently, porting ArceOS to a new hardware platform requires modifying the code in axhal.

We would like to remove the platform-related code from axhal and introduce them as extern crates, so that the main repository does not need to be modified when adding a new platform support. These crates can also be more easily reused by other projects.

Current progress:

• Introduce better config system. (Refactor config system with axconfig-gen #214)
• Extract CPU-related code from axhal. (https://github.com/arceos-org/axhal_crates/tree/main/axhal_cpu)
• Extract platform-related code from axhal. (https://github.com/arceos-org/axhal_crates/tree/main/platforms)