Compiling xgboost with NDK failes for armeabi-v7a and x86, due to the implicit cast on this line of c_api.cc:
out = new std::shared_ptr<DMatrix>(dmat->Slice({idxset, len}));
By adding explicit cast it will be solved:
out = new std::shared_ptr<DMatrix>(dmat->Slice({idxset, (std::size_t) len}));

cmake version: 3.16.3
ndk version: 21.3.6528147