When Clang is used as a cross compiler, it should respect the target
platform default. Defaulting to a configure-time CLANG_DEFAULT_STD_C/CLANG_DEFAULT_STD_CXX
does not make much sense.
When Clang is used as a single-platform compiler, we should use Clang
configuration file (https://discourse.llvm.org/t/configuration-files/42529
The Gentoo needs from D34365 has been satisfied by a configuration file.