The build was previously failing with `error: unknown command: -print-file-name=libstdc++.a`
because the command invocation was
`zig -print-file-name=libstdc++.a`
instead of
`zig c++ -print-file-name=libstdc++.a`
note: .cxx_compiler_arg1 = "" instead of undefined to avoid breaking existing setups without requiring to run cmake again.
Detect system libcxx name with the CMake build system and convey that
information to build.zig via config.h so that the same system libcxx is
used for stage3.
This undoes the hacky search 901457d173 .
closes#17018
