Define CppRuntime_Clang when building for Android

[s-ludwig]

With the current switch to NDK 26+ it should now be safe to assume the Clang C++ mangling/runtime.

The background for adding this is that the BindBC bindings currently error out when building for Android, because they require a C++ runtime version in order to potentially perform some manual symbol mangling.

[s-ludwig]

ld.lld: error: duplicate symbol: std::bad_alloc::bad_alloc()
defined at new_.d
new_.o:(std::bad_alloc::bad_alloc()) in archive /home/runner/work/ldc/build-cross-libs/lib/libdruntime-ldc.a
defined at stdlib_exception.cpp
stdlib_exception.cpp.o:(std::bad_alloc::bad_alloc()) in archive /home/runner/work/ldc/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/arm-linux-androideabi/libc++abi.a

How is this supposed to work? It seems like this is always defined on the D side, but for some reason errors out for the Android libc++ - should this be changed in DRuntime to only declare the constructor?

[kinke]

How is this supposed to work? It seems like this is always defined on the D side, but for some reason errors out for the Android libc++ - should this be changed in DRuntime to only declare the constructor?

The ctor should most likely be marked extern(D), as done for the other exception-derived classes in https://github.com/dlang/dmd/blob/b84fa4709f2061b60c2fd398a01964370cbb12c8/druntime/src/core/stdcpp/typeinfo.d#L185-L199.