System.Native: Add deployment target version for OSX (#42412)
System.Native: Add deployment target version for OSX (#42412)

As noticed in https://github.com/dotnet/corefx/pull/42408 we don't pass any minimum OSX target version to CMake.
This means it falls back to the host version, e.g. if you build on Mojave it would pass `-mmacosx-version-min=10.14` to the compiler.

.NET Core's minimum supported OSX version is 10.13 so pass that to CMake.
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