We can not use nativesdk variants of libcxx and compiler-rt yet when
compiling nativesdk-clang because, it will need this compiler to build
them, so solve this catch-22, since we do not use the runtime built
during compiler builds, use libgcc/libstdc++ to pass cmake tests
during configure, this should be fine as it will be not needed for final
builds where nativesdk-clang will be used, it can still default to llvm
runtime on SDK host

Signed-off-by: Khem Raj <raj.khem@gmail.com>