refactor: replace embedded arceos with submodule (customized for tour & exercises)

[langqiuyi-77]

What Changed

• Removed the embedded arceos/ directory from this repo.
• Added arceos as a Git submodule pointing to a forked version based on the upstream arceos.
• All previous patches to arceos have been ported and committed to the submodule repo directly.

Modifications Made to arceos

The customized arceos submodule contains the following changes compared to upstream:

1. Memory Config: Added MMIO mapping for pflash in platform memory layout.
2. API Changes:
   • Updated axmm::new_user_aspace
   • Updated UspaceContext::new
     These changes enable compatibility with:
   • tour/m_1_0, m_1_1, m_2_0, m_3_0, m_3_1
   • tour/h_1_0
   • exercises/sys_map
   • exercises/simple_hv
3. Console Output:
   • Downpatched the console output mechanism to use legacy SBI instead of console_write.
   • This ensures compatibility with riscv_vcpu, which expects legacy SBI console behavior.

How to Use

After checking out this PR:

git submodule update --init --recursive