Change SIM boards that uses libcxx to use Clang.

[Barto22]

This commit switches SIM board builds from GCC to Clang and migrates
the C++ runtime to LLVM's libc++ and libc++abi. This PR is related to: #17826 and #17849

Summary

Recent C++ code additions require features provided by modern C++ runtimes
and standard libraries. The GNU toolchain versions currently available in CI
and common Linux distributions (GCC < 15) lack several required C++ standard
library capabilities needed by the latest libc++/libc++abi.
To ensure continued progress on C++ support in NuttX, this change:

• Switches SIM builds to Clang as the primary compiler
• Adopts LLVM's libc++ and libc++abi as the default runtime/standard library
• Updates related flags and linker options for compatibility

This brings the SIM toolchain in line with modern C++ requirements and avoids
blocking future features on GCC version availability.

Impact

• SIM board builds now default to Clang + libc++/libc++abi
• Improved support for modern C++20/C++23 features
• No impact on embedded architectures/toolchains (limited to SIM boards)
• No ABI impact for non-SIM boards
• CI builds gain broader warning coverage and better diagnostics
• No documentation updates required at this time, but users building SIM
  locally may need Clang + libc++ installed

Testing

Test matrix executed across Linux CI:

• SIM builds validated with Clang + libc++/libc++abi
• Full build completed without regressions
• ostest executed successfully under the SIM target
• Additional C++ examples confirmed to compile and run correctly

Host: GitHub Actions CI (Ubuntu Linux)
Compiler: Clang 21 (LLVM libc++ 21.1.8)
Prior GCC versions tested (< 15) confirmed incompatibility with modern libc++

No regressions observed in SIM functionality.

[raiden00pl]

this way you break the test env for all users who have gcc-15 support on their hosts and don't use clang which I think is the default behavior for nuttx users. If you want to fix errors in CI, you should change the compiler in tools/ci/testlist/sim-XX.dat like its done for example here:

nuttx/tools/ci/testlist/arm-04.dat

Line 11 in a938926

/arm/lpc54xx,CONFIG_ARM_TOOLCHAIN_CLANG

[Barto22]

this way you break the test env for all users who have gcc-15 support on their hosts and don't use clang which I think is the default behavior for nuttx users. If you want to fix errors in CI, you should change the compiler in tools/ci/testlist/sim-XX.dat like its done for example here:

nuttx/tools/ci/testlist/arm-04.dat

Line 11 in a938926

/arm/lpc54xx,CONFIG_ARM_TOOLCHAIN_CLANG

Done

[xiaoxiang781216]

@Barto22 please sqaush your intermediate patch.

[Barto22]

I'm updating Dockerfile in this PR: #17849 . After merging this changes: #17849 there will be no need to use clang for tests sim boards because there is updated gcc and g++ to version 15.2. So I'm closing this PR.