[Issue 24623] Rename version CppRuntime_Clang/Gcc to CppRuntime_libcxx/libstdcxx.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24623

--- Comment #3 from johanengelen at weka.io ---
libcpp is the GCC C PreProcessing library).

libstdc++v3 refers to itself as (lib)stdcxx in code.
libc++ similarly.

Changed title of bug report: cpp --> cxx

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