[libcxx] Duplicate symbol with libcpmt.lib for static libc++ builds

I understand parts of libc++ is implemented by linking with the MSVC's standard C++ library implementation. As far as I can tell, however, for static builds this requires linking against `libcpmt`, which causes duplicate symbols when linking executables.

```
lld-link: error: duplicate symbol: int __cdecl std::uncaught_exceptions(void)
>>> defined at D:\a\_work\1\s\src\vctools\crt\github\stl\src\uncaught_exceptions.cpp:9
>>> libcpmt.lib(uncaught_exceptions.obj)
>>> defined at libc++.lib(exception.cpp.obj)
```

Is this a known/expected issue?