arch/risc-v: Refactor LLVM CPU type handling in Toolchain.cmake

[no1wudi]

Summary

• Replace direct string comparisons with regex pattern matching for ARCHCPUEXTFLAGS
• Change from using LLVM_CPUFLAGS list to setting LLVM_CPUTYPE directly
• Simplify CPU type detection logic while maintaining same functionality
• Use more consistent string variable naming convention

This change makes the CPU type detection more flexible and maintainable while keeping the same behavior for supported RISC-V configurations.

Impact

build system only

Testing

GitHub CI and apache/nuttx-apps#2487

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR appears to meet the NuttX requirements, although it could be more thorough. While it addresses the core elements, providing more details in the "Impact" and "Testing" sections would significantly improve clarity and confidence in the changes.

Specifically:

• Impact: While it states "build system only," it lacks details. How does the build process change? Even if seemingly minor, articulating the change (e.g., "Removes the need for an explicit list of CPU flags, simplifying the configuration process.") provides valuable context.
• Testing: Referencing a related PR in nuttx-apps is good, but including specific test results (even summarized) directly within the PR is essential. Simply stating "GitHub CI" is insufficient; mentioning successful builds for specific architectures/configurations strengthens the validation. The "Testing logs before change" and "Testing logs after change" sections are empty, which is against the stated requirements. Even if the behavior is identical, showing some output demonstrating this would be beneficial.

By adding these details, the PR would more robustly adhere to the guidelines, making review easier and increasing confidence in the proposed changes.