BuddyDeepSeekR1 supports riscv cross compilation and update LLVM construction method

[jiaan-clone]

Summary

• BuddyDeepSeekR1 supports riscv cross compilation
• README.md -> How to cross compile RISC-V target on x86_64 machine
• Update LLVM construction method in docs/RVVEnvironment.md

Checklist

• [ OK] The code builds successfully
• [ OK] Existing tests pass
• [ OK] New tests are added where appropriate
• [OK ] Code follows the project coding style
• [ OK] Documentation is updated if needed