Implement a basic nanoSBI framwork

Remove arch component for simplicity

Remove RVM component for simplicity

Update rust toolchain

Update .gitignore

Implement H-extension support detection.

Build a RVM code framework.

Implement a basic timer and interrupt handling mechanism.

Update README.

Fix the value of `PHYS_MEMORY_BASE`.

Basically adapt to riscv64 arch.

Initial commit.