arch/arm: Add LLVM configuration to CMake

[no1wudi]

Summary

Added LLVM-specific configuration variables to ARM architecture CMake files:

• LLVM_ARCHTYPE for architecture variant (thumbv6m, thumbv7a, etc)
• LLVM_CPUTYPE for CPU target (cortex-m0, cortex-a5, etc)
• LLVM_ABITYPE for ABI (eabi/eabihf)

Impact

New CMake flags for ARM

Testing

Tested with apache/nuttx-apps#2487

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR appears to meet the basic NuttX requirements, but is missing some key information. It provides a summary of the changes and identifies the impacted area. The testing section links to a related PR in nuttx-apps, which implies testing was performed, but lacks specific details.

Here's what's missing and needs to be addressed to fully meet the requirements:

• Summary: Needs more detail on why these changes are necessary. What problem do they solve? What benefit do they provide? What was the previous behavior, and why was it inadequate?
• Impact: The impact section is too brief. While it identifies new CMake flags, it doesn't explicitly answer the specific impact questions (user impact, build impact, hardware impact, documentation, security, compatibility). Even if the answer is "NO", it should be explicitly stated. For "build impact," since there are new CMake flags, it's definitely a "YES" and requires more description about how the build process changes with these flags.
• Testing: Linking to a related PR is insufficient. The PR should contain its own testing logs demonstrating the behavior before and after the change. The linked PR may provide context, but the PR itself should be self-contained in terms of verification. Also, the build host and target details are missing.

In short, the PR provides a starting point, but needs to be fleshed out with more specific details to be considered complete.