nuttx/libxx: Simplify compilation warnings for passing CXXFLAGS.

[cuiziwei1]

Summary

Remove the judgment of GCCVER and add CXXFLAGS directly. If there is a problem that the compiler cannot recognize the compilation options, add it separately.

Impact

Testing

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not meet the NuttX requirements.

Missing Information:

• Summary:
  • Why is this change necessary? What problem does it solve?
  • What functional part of the code is being changed (be specific)?
  • How does removing the GCCVER judgment and adding CXXFLAGS directly address the issue?
• Impact:
  • You've left all impact sections blank. Address ALL of these:
    • New features?
    • User impact?
    • Build process changes?
    • Hardware (arch, board, driver) changes?
    • Documentation needs?
    • Security implications?
    • Compatibility concerns (backward, forward, interoperability)?
    • Anything else to consider?
• Testing:
  • Insufficient detail:
    • Specify the exact build host operating system, CPU architecture, and compiler version.
    • Specify the exact target architectures, boards, and configurations used for testing.
  • Missing logs: Provide actual testing logs from before and after the change.

Recommendations:

1. Explain the "Why": Clearly articulate the problem that this change solves. Is there a bug report or feature request this relates to?
2. Be Specific: Don't use vague terms. Name the specific files, functions, or configuration options being modified.
3. Complete Impact Assessment: Think through all potential consequences of this change, even if they seem minor.
4. Provide Detailed Testing Information: Give reviewers the information they need to reproduce your testing environment and verify your results.