libs: Add support for latest LLVM LIBCXX and LIBCXXABI version 21.1.8

[Barto22]

Before merging and running this PR, this two must be merged: #17849 and #17850

Summary

This PR updates the build environment to use the latest Ubuntu 24.04 LTS base image and upgrades the GNU toolchains to:

• ARM GCC 15.2.rel1
• RISC-V GCC 15.2.0

All SIM board configurations that enable LIBCXX are now built with clang, as GCC < 15 lacks required builtin features for C++23 and libc++.

This continues the integration effort for LLVM libc++ and libc++abi (v21.1.8) in NuttX, enabling modern C++23-compatible standard library components including new language features and exception handling.

Impact

• Modernizes CI and build environment to current LTS
• Improves ARM/RISC-V toolchain compatibility
• Ensures SIM + libc++ builds consistently via clang
• Existing GNU libstdc++ and non-libc++ workflows remain unaffected

Current status:

• Runtime / ABI: basic C++ exception handling is functional
• Compatibility: legacy C++ toolchains are unaffected
• Build systems: validated via make and cmake
• Forward support: establishes a solid baseline for C++23 and libc++ work

No impact to existing boards or applications unless enabling libc++.

Testing

Host environment:

• Linux x86_64 (Ubuntu 24.04 LTS Docker image)

Boards / Configurations tested:

• stm32f4discovery:testlibcxx
• esp32c3-devkit:testlibcxx

Toolchains:

• ARM GCC 15.2.rel1
• RISC-V GCC 15.2.0
• clang (for SIM + libc++ targets)

Procedure:

• Built with both make and cmake
• Verified exception handling
• Validated libc++ functionality
• Confirmed non-libc++ builds remain functional

Status:

• libc++ functionality verified
• Exception handling working
• No regressions detected

Logs and additional validation data are available on request.